Generally, it has been going off of the MP cost of the spell, slightly elevated. A Resist would cost you somewhere between 6-10 build-points, whereas as Windwalker would cost 6-8 build-points.
It has been generally allowed to build the item in slight violation of the magic item creation rules, meaning multiple skills or spells in a single item.
Each case is different, and is best discussed with James directly, keeping in mind the generalities I mentioned.
/* OOG
* Rob G.
* Rules Marshal, Margrave
*/