I have a player using the inscribe magical tattoo feat and driving me up the wall with new rules to make these callings on.
We've figured out a bunch already- (Thanks to you lovely people!) but I find myself hitting another wall.

He would like to create metamagic tattoo's on his hands.
Hypothetically he can 'wield' two metamagic tattoos while still casting spells since technically his hands are free. I'm okay with that.
It seems he can create these tattoo, without having the actual metamagic feats in question by simply adding a +5 to the craft tattoo DC. (as OP as that sounds). I am okay with that.

Somehow though, the price of this tattoo is only double (for being slotless)what a similar rod would be, and is a free action to use.

Could someone please help with the pricing on this?

Seems like having a 'hand' item acting 'slotless' is worth a lot more then having a neck item being slotless- I mean come on! They're hands! They're in high demand.
And as far as I can tell- activating tattoos will be a standard action. Just because it's a metamagic tattoo does not automatically make it free... there should be an additional price increase, correct?

p.s. We've already discounted the cost to make tattoos, since he sold me on the idea that they are not 'resell-able" and therefore not worth as much...
So an extend metamagic tattoo (still allowing his hands to be free and using a free action to activate) would cost him only 2100 to make.
Only 600 more then a regular metamagic rod of lesser extend...

Slotless: Items not worn or carried in one of the above slots are called “slotless” items. Sometimes these items take the form of trinkets, like figurines of wondrous power. Other times they are larger items, such as the carpet of flying. Typically the possession of such an item is enough to gain its benefit, but sometimes one must manipulate and activate the item.

(emphasis mine)

This is why slotless items are worth extra; you do not need to wield them or hold them or anything in order to use them. So he could actually scribe tattoos all over his body for each slot and be able to choose which tattoo to use when necessary; they don't even need to be in the hand slot.

Transfer Tattoo makes tattoos transferable. Discounts would have to come from alignment, class, and/or skill restrictions (unless you want to house rule stuff).

Item Requires Skill to Use: Some items require a specific skill to get them to function. This factor should reduce the cost about 10%.

Item Requires Specific Class or Alignment to Use: Even more restrictive than requiring a skill, this limitation cuts the price by 30%.

I've seen debate over whether these should decrease costs or not. By RAW, I would say they decrease cost, but you can interpret these things how you want.

Caster's Tattoos use a swift action to activate, but they're a bit different from what it sounds like you're going for.

This is what the rules have for metamagic rods (and, by extension, a metamagic tattoo):

Possession of a metamagic rod does not confer the associated feat on the owner, only the ability to use the given feat a specified number of times per day. A sorcerer still must take a full-round action when using a metamagic rod, just as if using a metamagic feat he possesses (except for quicken metamagic rods, which can be used as a swift action).

Metamagic rods (which is what the effect of the tattoos sounds more like) function the same way as the feat. In the case of someone using a metamagic rod, it makes the act of casting a spell a full-round action if they are a spontaneous caster. In the case of a prepared caster, it has no effect on casting time. Activating the metamagic rod (and a metamagic tattoo, from my understanding) would not require an action. It's effects would be dependent on the type of caster.

Also:

A caster may only use one metamagic rod on any given spell, but it is permissible to combine a rod with metamagic feats possessed by the rod's wielder.

. So, only one tattoo can be used on a spell at a time.

TL;DR:

The cost should be double unless the tattoo is given alignment, class, and/or skill restrictions, because it IS transferable with a spell.

Tattoos (and rods) both take no action to activate. Spontaneous casters are required to cast the spell as a full-round action unless using quicken spell.

Only one rod (or one tattoo) can be used at a time.

Essentially, the tattoo just means he doesn't have something in his hand. It's not a big deal if the cost must be doubled (also, if tattoos can have discounts for restrictions, so can rods; he should definitely be spending double the normal amount for the tattoos)

Hope this answers all your questions.

It does indeed! Thank you.