So just got an item that gives a +6 to all craft checks and was trying to figure out how much it is worth. I see that the Circlet of Persuasion(4,500gp) gives a +3 Competence bonus to all Cha-based skill checks so was going to use that as the template for the value, but after trying to figure out what rules apply to it I ran into alot of dead ends.
Skill Bonus(Competence) cost is valued at (Skill Bonus Squared)*100. So 3*3 = 9 * 100 = 900. Obviously not what the Circlet is using. Since there are 7 Cha-based skills(I know this isnt all it applies to) i tried to multiply it by 7 so 7 * 900 = 6300. Again not the right amount. Though about it being treated as multiple effects but that just multiplies 6 of the 7 skils by 1.5 which just makes the cost higher and still not the right amount. What crafting rules are used for this item?
Bonus squared * 100 is 900. 4500 is 5 * 900. So, obviously, someone either missed a few skills, or gave it a discount.
Not all items are priced as per formula. They (the Devs) may have started with the formula as a baseline and then altered it from there based on some logic.
In this case I could see the logic being: not everyone is going to use every skill so lets reduce the price downwards a bit to account for that.