depends on the live wallpaper. i use a live wallpaper not yet on the market and it uses less than .25% of the battery and causes no noticable lag at all. i have used several others that eat a considerable amount (mario for one), that one ate something like 5+%. it all depends on what the wallpaper is doing (mario is doing alot behind the animations) and how well the wallpaper is coded, for example that it handles the visable and pause events correctly so that it doesn't run when not visable.
for example if the live wallpaper is using gps, or a host of other things including data etc... it will be more intensive and beat up the battery, like wise if it is running at a high frames per second takes more from the phone. like everything it isn't "live wallpapers" it is how well it is coded and how much it is doing. Think of live wallpapers as an app or technically it is a service that is running. for example i know if i run google maps navigation it is going to eat 10% of the battery every hour i run it, now that is a battery hoggin app. not every app uses that much battery, live wallpapers are the same, some sip the battery others chug it like a frat-boy at a kegger
