I'm playing an Asmodean Advocate in a pbp game and the party just reached 3rd level. I'm taking Craft Wondrous Item and the game is pretty lean WBL-wise.
One of the low-cost items that I can craft that will pay off for my PC is Sinner's Wage. He tends to have an overpowering personality (sky-high Bluff & Diplomacy) and we've had a lot of success with compulsion spells, so anything that bumps up the DC for charm/compulsion is worth the gold.
Since the cost for the item itself is so low, I am wondering what the RAW cost breakdown is to craft Sinner Wage at a +3 or +4 bonus to the Charm/Compulsion DC instead of the normal +2?
(I know using it this way drains it for a week.)
There isn't such a guideline in RAW. I would look for debuff spells that do a similar thing, then see if there are greater versions of said spell and see how the math plays out from there.
That is where the crux is. The closest thing I can find are Feats, Spell Focus and Greater Spell Focus (which would give a total of +2 to the DC of the school), but Feats are so wide and varied there is no mechanic for, "cost of a Feat is A x Bgp."
The next closest one can find is figuring the opposite of Save Bonus, which is out there regarding cost (either *1000gp or *2000gp), but the root item is much less due to only counting against one school and works one time a week.