Any spell currently in effect. Same as Permanency: you can cast it on someone else's spell, as long as it's currently functional.

I can't think of a better way to calculate an XP cost, though. Perhaps ((SL * CL) + 2) * 100? (SL * CL * HD * 100)? (SL * SL * CL * CL * 100)? (SL+2 * CL+2 * 100)?

Also, a True Permanencied Greater Invisibility would cost 4 * 4 * 7 * 100 = 11,200 xp