Offering closure
I spoke to one of the other DMs and we agreed on what the item is, and what it costs:
Mantle of the Erlking
This mantle of mithral chain and sapphire grants the wearer of at least 6th level the Leadership feat, as well as granting a +2 Enhancement bonus to their Wisdom and Charisma. Uses Cloak slot.
Restrictions: This item works only for Fey (including Elves and Elf-blooded), and may only attract Fey/Elf cohort and followers.
Item price: 11,550 gp.
Cost to create: 5,775 gp, 462 Exp.
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We came up with this price using the following formula:
Leadership Feat cost 10,000
+2 Wisdom 4,000 +50% for wrong slot, x 75% for second related power.
+2 Charisma 4,000, x 50% for tertiary related power
That totals 16,500 gp.
The racial restriction, per the SRD, cuts the price/cost by 30%. We had some discussion on whether these were "related powers" or not, but concluded that they were because they are mental stat boosts tied to being a good leader.
Tying Charisma to Leadership was easy.
Tying Wisdom to Charisma isn't hard either.
Tying them all together was the part we questioned, but since there are few if any guidelines in the books on that point, and we were reasonably able to "theme" them, we went for it.
Anyway, I just wanted to share what we concluded in the end. (I'm sure someone will disagree with this.)