You're basically in GM-fiat territory, so ask the GM.
If they do allow it you're probably looking at this part of the item creation territory:
If the item is one that occupies a specific place on a character’s body, the cost of adding any additional ability to that item increases by 50%. For example, if a character adds the power to confer invisibility to her ring of protection 2, the cost of adding this ability is the same as for creating a ring of invisibility multiplied by 1.5.
Going from a +1 weapon to a +2 weapon would normally cost 6000gp (8000gp for a +2 weapon minus 2000gp for the +1 weapon that you already have). If you craft it it'll cost half this amount.
If you're increasing those costs by 50% you'd pay 9000gp, or 4500gp to craft yourself to go from a +1 enhancement bonus to a +2.
Since Rods tend to be very specific and powerful magic items You'd definitely have to ask the GM though.
Yeah that's my issue to. I can understand that it COULD go up but is it 50%? At all?
Just curious. There are few rods that get it anyways.
From what I can see there ARE some rods that add in weapons qualities, they simply require craft arms and armour.
The one exception is the fairy nimbus rod which according to Nethys requires craft wonderous. Likely just badly written.
But a few yeah, just need the feat to add in more. So it could be done? Just the cost of it. I'm not exactly an expert of breaking down costs.
But I could see liberators rod and bellowing rod add in things like thundering and merciful.
I don't think it's intended. There is the Spellsword spell that allows you to combine a rod and a weapon temporarily, so it's unlikely that a cheap or easy rod as weapon enchantment is intended.
That said, I don't care much for the need to juggle items anyway, so I'd be happy to let someone merge their rod and a weapon without additional costs.
For classes like the Magus it is a clear advantage. Especially if you make a weapon that acts as a rod.
As a minimum, it allows making AoOs without the need to extract a weapon, so useful for people that go often in melee.
When making items the 50% surcharge is added to the less costly of the merged items, so, as long as you don't try mixing item that are too outlandish the cost isn't so high.
The effect of combining a weapon and a rod is so variable that the GM should evaluate it each time.
enhancing non-standard magic items is a tricky business. You and your home game GM will have to come to an agreement on the process and cost. I'd agree that it will generally cost 1.5 times the standard cost (see magic item creation table).
The hard questions in lowering the cost are;
1) how is the base attack enhancement bonus costed in the original item (with some reverse engineering accounting) if any. Some rods/staves are designed AS weapons.
2) can the power/special ability of the rod be activated and used offensively if a target is struck using a melee attack with the rod.
3) is this similar to adding attack bonuses to a shield.
I'd agree that the spell Spellsword Wiz3 is probably the more cost effective way to go and includes some extras. See also Weaponwand Clr1 ('cause Wiz 1 would have been too obvious). You *could* just add a once per day usage of the spell to a weapon using std activation and use any rod.
Well I was actually looking at a rod with Asmodeous deity on a rogue.
So a lot of spell casting options I'm meh about.
Rods CAN be maces. The ones I'm looking at certainly are. I'm more just thinking "is this stuck at the plus 1 the book gives it or can it go higher."
I'm not trying to add in weapon enhancements to a rod, I'm trying to enhance the weapon enhancements already given and the cost of it.
I'm ok with paying 50% more.
There are no rules for enchanting a Rod of Staff as a weapon. I believe most of the Rods and Staves that can be used as a weapon are not priced like a weapon enchant. Especially the hot mess that is a Rod of Lordly Might. When your weapon can transform between being a +2 enchant, or a +3 enchant to a +4 enchant...how much is it worth for your weapon to have different forms and yet be clearly inferior?
While it makes sense to be able to improve these 'magical weapons' Rods and Staves are nonsensical items. Don't try to apply logic here, the designers of this vast hodgepodge of items obviously didn't. The very fact that there are no single spell staves or no clear definition of what a rod does should make that apparent.
So you are totally in GM's judgement here. Personally I don't think its a good idea to let the enhancment bonus on a Rod or Staff be advanced. These are suppose to be odd, unique and quirky items. Not something reliable like an actual weapon.
... I'm ok with paying 50% more.
Talking to the public at large will give you some opinions and options, and an idea on practicality.
Really you need to talk to your home game GM. He and you need to come to an agreement.
With "named items" and only the 150% option, in PFS this is going to be a NO. Even paying 150% for a numerical bonus is going to raise uncertainty and PFS GMs are generally not that familiar with the magic system.