Modeling off shocking grasp works, since it's a touch thing. I'd say it expends the spell level.
Arcane/Eldritch/Mystic Shock (Su): A magical leech can expend spell levels as an immediate action to shock any creature touching it. This does 1d6 points of electrical damage for the first spell level expended plus an additional 1d6 points electrical damage for each additional two spell levels the leech expends. (Care for a save for half?) (Allow the leech to make a touch attack?)
Well, I'm open to discussion about the strength of the shocking touch as well as its cost in levels (I like what you have there quite a bit actually, though maybe we can streamline it a bit). But I really don't think it needs to parallel the psionic leech so much that it needs a blast ability.
How about we jus take your version of the shocking touch and leave it at that? It can be used defensitively if it's an immediate action, and it can be used offensively if the leech happens to be grappling its victim anyway; it doesn't need a touch attack.
Yes, that seems great to me!

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.