How much would +1 Elven Chain with a +4 competence bonus to diplomacy when dealing with elves cost?
Elven Chain=5150 gp
+1=1000 gp
+4 Skill bonus (competence), but only with one race=??
How much would +1 Elven Chain with a +4 competence bonus to diplomacy when dealing with elves cost?
Magic Item Gold Piece Values: "Many factors must be considered when determining the price of new magic items. The easiest way to come up with a price is to compare the new item to an item that is already priced, using that price as a guide. Otherwise, use the guidelines summarized on Table: Estimating Magic Item Gold Piece Values."
The real answer is "Ask your GM."
My opinion:
Multiple different abilities: Multiply lower item cost by 1.5
Elven chain: 5,150 gp
+1 armor enhancement bonus: 1,000 gp
Assuming your GM rules that you can enchant specific named items, that makes the +1 elven chain cost 6,150 gp.
A Belt of Dwarvenkind is basically what you want, only for elves.
So putting the abilities of a belt of elvenkind onto +1 elven chain armor would mean you multiple the lower cost by 1.5 then add the more expensive one.
(6,150 x 1.5) + 14,900 gp = 24,125 gp.
That would get cheaper if you removed some of the bonuses.
+2 to Constitution (or dex, for elves) is worth 4,000.
+2 vs Poison is worth 2,250 gp. (elves get enchantment instead)
Darkvision is pretty expensive. (Low-light, probably balances out poison vs enchantment)
A Circlet of Persuasion is +3 to all Cha checks for 4,500 gp. So a slightly bigger bonus that only applies to elves should probably be somewhere around the same price. So if you want to eliminate the penalty for non-elf races, say it's about 5,000. That makes the +1 elven chain of elf persuasion around 13,000 gp.
You'll get completely different results if you use the formula.
Skill bonus (competence): Bonus squared x 100 gp
(4x4)x100 = 1600 gp.
1600x1.5 = 2400 gp
2400 + 6150 = 8,550 gp for +1 elven chain with +4 competence bonus to all diplomacy checks.
But because that same formula makes the circlet of persuasion cost 900 gp instead of 4,500 gp, we can tell that we need to use the existing item as a guide instead of the formula.
I like your thinking with the Circlet of Persuasion (5,000), but this is just to diplomacy so it should be discounted. There are 7 Cha based skills, so 1/7 the cost?
Call it 700 gp?
Armor special abilities giving skill bonusses cost 'bonus squared * 1.5', as seen amongst others on the shadow enhancement.
That puts the base price, as Grick mention, at 2400 gp.
I would be inclined to grant you the 30% discount, similar to "only usable by certain class or alignment". While it can be used by non-elves that bonus cannot be used on non-elves.
This reduce the price to 1680 gp.
While it is fair to consider getting a bonus against anyone by paying slightly more. But in the specific case, I would expect that a bonus towards elves is actually going to be useful. Due to that it should cost slightly more than pocket change.