There are actually spells that already grant the user the ability to lifesteal in d20 (Vampiric Blade comes to mind). Perhaps you could consider those in mind when you make up the price?
I searched the spell database for "Vampiric Blade" and nothing came up. Where is it found?
Currently, I feel that Vampiric Touch is the closest spell that has a lifesteal effect similiar to what the item grants, thus using it as the pre-req spell. As for price, I'm thinking 10,000 right now. It's a good item, but has a fairly potent drawback.