So I'm running Carrion Crown, and one of my players is a heavy mad-scientist type character (his idea of a lockpick, which we call the locktopus, is a betentacled construct/wondrous item that tries to crawl away from you and hide in a corner if you use it too many times). He recently submitted to me this idea for a construct he'd like to build, and as it's quite complex I was hoping to get some advice on how balanced it is. Quoted below is the entire writeup as he sent it to me.
* Tiny Construct, not battleworthy, that looks like a bipedal taxidermy project involving a dog, a monkey, a woodstove and a lot of rubber tubing.
* Has a few utility spells like detect magic, scoop and spark (for use in the lab), plus crafter's fortune and identify a total of 5/day. The creature is a true spontaneous caster with its creator's spell list, but never gains more spells known nor any more spells per day. Being a caster is the construct's major feature, and a necessary one to make it capable of crafting.
* Can assist with crafting in three ways:
A) It has a Lab Assistant ability that permits it to take over half of its creator's daily crafting time, freeing the master to do other things. This application works for all magical crafting and for any mundane crafting in which the wichtelmaschine is proficient (in this case, alchemy, clockwork and one more TBD).
B) The wichtelmaschine can also craft independently. It has Craft Wondrous Item and three mundane crafting skills, all of which it can use unattended at its master's command. The creature is a pretty competent crafter, but nowhere near Milos' level. This application doesn't benefit from any of the master's abilities.
C) The wichtelmaschine has the Cooperative Crafting feat, which permits it to assist its master on a project to double the daily gp value of crafting he can do. This does require the wichtelmaschine to have the relevant item creation feat or craft skill; it doesn't work for all magical crafting as Lab Assistant does. Cooperative Crafting is mutually exclusive with Lab Assistant.
* The wichtelmaschine's major weakness is that it can go "berserk" during extended crafting and become obsessed with a side project, making it useless to its master until the it is permitted to build its random device. It's mostly flavor, but also a break on leaving the construct at home to work on months-long crafting projects while the party adventures.
* The creature is CR 3, but is priced as a CR 4 construct since its battle stats don't adequately reflect its usefulness. Its creation cost is 4000 gp, making its price somewhere around 8k.
Wichtelmaschine
This tiny, bipedal monstrosity has a dog's head and a body stitched from other several other small animals. Tubes running with fluorescent fluids criss-cross its body, and it moves with the halting precision of a mechanical clock.
Wichtelmaschine CR 3
XP 400
Neutral Tiny construct
Init +1; Senses darkvision 60 ft., low-light vision, scent; Perception +0
DEFENSE
AC 15, touch 13, flat-footed 14 (+1 Dex, +2 size, +2 natural)
hp 27 (5d10)
Fort +1, Ref +2, Will +1
Defensive Abilities construct traits
OFFENSE
Speed 20 ft.
Melee 1 bite +5 (1d4–2)
Space 2-1/2 ft.; Reach 0 ft.
Sorcerer Spells Known (CL 5th; concentration +7)
1st (5/day)—crafter's fortune, identify
0 (at will)—detect magic, mending, open/close, scoop, spark
STATISTICS
Str 6, Dex 12, Con —, Int 15, Wis 10, Cha 12
Base Atk +5; CMB +1; CMD 12
Feats Craft Wondrous Item, Skill Focus (Spellcraft), Cooperative Crafting
Skills Craft (any three possessed by creator) +9, Perception +0, Spellcraft +10, Stealth +9
Languages Common or builder's primary language (cannot speak); rapport
SPECIAL ABILITIES
Spells
A wichtelmaschine casts spells as a 5th-level sorcerer, but with a class spell list identical to its creator's. It does not gain any additional abilities, such as a sorcerous bloodline, and it never gains additional spells known nor spells per day.
Rapport (Ex, Su)
Though it cannot speak, a wichtelmaschine's grunting verbalizations are perfectly comprehensible to its master. Even when this supernatural ability is not functioning, the wichtelmaschine retains an extraordinary ability to make its general meaning known to its master through grunts and body language, thanks to their fundamentally similar psyches.
Lab Assistant (Su)
Wichtelmaschines have a special link to their masters in the laboratory. When its master is engaged in mundane or magical crafting, a wichtelmaschine can use this ability to free its master from work for up to half of the total crafting time he or she would otherwise have to spend each day. For example, a wizard brewing a standard potion of Cure Light Wounds (which takes 2 hours to brew) could enlist her wichtelmaschine's assistance and spend only one hour on the project, leaving the wichtelmaschine to attend the project for the other hour. Likewise, a cleric forging holy symbols could set her wichtelmaschine on the task and spend only four hours per day at the forge, yet still make the usual weekly Craft check at the end of the week. An adventurer working at half efficiency due to being in the field may offset this penalty by using a wichtelmaschine, but gains no other benefit.
Using a wichtelmaschine in this way requires careful arrangement of tasks and some oversight, so it is impossible to "double up" and achieve more than 8 effective hours of crafting per day. Masters who wish to speed up crafting in this way may command their wichtemaschines to craft independently, of course.
A witchtelmaschine need not possess the relevant item creation feat to assist its master in crafting a magical item; however, it must possess the relevant Craft skill to assist its master in mundane crafting. Wichtelmaschines with the same master will not cooperate. Any task to which they are assigned will make no progress, whether or not they use this ability.
Inspiration (Ex)
Every day that a wichtelmaschine spends working on a single crafting project (either assisting using its Lab Assistant ability or crafting independently using its own feats and skills), it has a cumulative 1% chance to become inspired. An inspired wichtelmaschine hits upon an idea for an object to build and loses interest in everything else; DMs should roll secretly on the chart below to determine the nature of the item. The inspired wichtelmaschine immediately ceases work on its current job, informs its master (giving its master a chance to take up the job and avoid losing all progress or ruining materials), and begins acquiring materials in any way that it can, from begging to stealing to cannibalizing its former creations. Once it has acquired materials, it begins building. Until it has finished its random project, a wichtelmaschine cannot and will not craft anything else, nor can it use its Lab Assistant ability. If its master decides to indulge it and take up the project, however, a wichtelmaschine will permit its master to craft the item and use Lab Assistant to speed the process.
The cumulative chance for inspiration resets every time the wichtelmaschine ceases work for a day or switches to a new project.
Random Inspiration
01-20 Choose a mundane item associated with one of the Craft skills the wichtelmaschine possesses. Roll randomly for the item's price on the table below, then select an item within that price range (or as close as possible, if no such item exists):
01-33 0-50 gp
34-66 50-200 gp
67-90 200-1000 gp
91-00 1001+ gp
21-99 Roll for a random Wonderous Item.
00 An elaborate suicide device costing 500 gp to build. The wichtelmaschine's brain has awakened and now craves death. Immediately upon completion, the device activates and irreparably destroys the wichtelmaschine.
Construction
A wichtelmaschine is a humanoid clockwork within a "skin" made of preserved hides from small mammal, usually with the head of a dog, fox or small bear. Tubes of flowing alchemical liquids run through the entire body, connecting to a glass case concealed inside that holds a humanoid brain. The materials to construct the body cost 500 gp all together.
CL 7th; Price 3000 gp
Construction
Requirements Craft Construct, animate dead, crafter's curse, crafter's fortune, detect magic, identify, mending, open/close, scoop, spark; Skill Craft (leather), Craft (clocks) or Craft (alchemy) DC 12; Cost 4000 gp.
I would consider revising inspire. It steals materials from things to build random things that might be good? This will either piss off players, npcs, and/or it might give him a slight gold leap. The entire thing seems to be pretty powerful, however since they have taken so much time thinking about it I would just give it to them after a inspire revision. Reasonably rewarding players for roleplaying is always a good thing.
Your best guide is going to be to use the rules for constructs given in the Mastering Magic section of Ultimate Magic. Note that the 1st special ability is free, the next two increase the CR by 1/2, and all others increase the CR by 1.
I would suggest comparing with a Soulbound Doll base to represent the intelligence required to have the feats necessary to create magic items and the limited spellcasting. This would also allow for the feat selection that was described. Note that the "berzerk" quality normally decreases the CR by 1, but in this case the drawback is so minor I wouldn't include that modifier, chalking it up to a swap for the vulnerability to mind affecting effects the soulbound doll would have.
All in all, I like the feel of Lab Assistant and Inspiration qualities. I would use the spellcasting as the free ability, but force your player to set those spells or use wands to add variability. I would bump the CR by 1/2 for the Lab Assistant ability, and give it the Master Craftsman feat for another +1/2 CR so he wouldn't have to be a full caster to craft. Then your player would just have to give the construct feats based on it's HD (base 3d10 for a soulbound doll, using the monster advancement charts to estimate CR increase for more HD).
I know this doesn't answer your question outright, but I hope it's a start. I'll look into things further if you'd like and give you my estimate on what the cost should be to make one of these.
As for balance, I really don't see the issue as far as combat. It will generally be a weak fighter more prone to hiding in a corner or tossing our magic items. As far as wealth, be careful with any crafter. I would say that he should never be able to have more than a 10% profit margin due to other costs such as finding buyers, maintaining a shop, etc. I'd even try to base this off expected wealth increase at each lvl, so as not to derail the game too much, but still allow the character to feel that it's a worthy investment.
Edit: I would also caution about the Inspire ability. I might allow the item created to be sold for a small amount (maybe even make up for the loss of materials if the previous project was ruined and was a good roll on the idea), but the chance of maybe getting a good random item is a bit too powerful IMHO.
OK, so here's what I've got so far:
3HD gives it a base CR 2 and 2 feats, 5HD increases that to CR4 and 3 feats
Small size decreases the CR by 1 (as per the Small template)
Intellegence, combined with limited spellcasting would be the "free" abilities.
Lab Assistant and the free Master Craftsman feat are each +1/2 CR bump, for a total of +1 combined.
Give it the Cooperative Crafter and Craft Wonderous feats as the ones gained from HD, plus one additional if he didn't mind the CR4 base to get an additional 2 HD and an extra feat.
Materials would cost 300gp (as per Soulbound Doll)
Revise the Inspire ability to make an item equal to a % roll times the 1/3 materials cost of the current item (the amount that would be lost if it was ruined). If you feel like being generous, you could increase this to 1/2 the material cost or give the construct the Deep Pockets ability from the Pathfinder Chronicler prestige class, which would fill the flavor I think the character is going for. Just be sure that the construct uses its HD, not the character's lvl.
Total CR becomes:
For base CR2 (3HD), final CR3, 2750 build cost (4800 market price)
For base CR4 (5HD), final CR5, 6550 build cost (12800 market price)
Of course these are all estimates, and to be safe you could always increase the initial costs and let the construct "pay for itself" to make up for any over adjustment by awarding your player extra gold/items over time.
Edit: As this is essentially a cohort, not just another NPC, I'd suggest letting your player make the construct using a point by system one step lower than he used for himself, but setting the Con modifier at 10 for those purposes (constructs don't have this modifier, but if you read the construct type entry in the bestiary, they treat it as a 10 for all purposes). Don't forget to add in the modifiers for being small though (+2 Dex, -4 Str, -2 Con, +1 Atk, -1 CMB/CMD, +4 Stealth, as per tables 2-2 & 2-3 on page 296 of the bestiary, though with the size change being reversed).
Edit: I had read the atk & CMB/CMD off the line for tiny, though I really should have known that what I was typing was wrong. It's correct now.