This is an interesting question, actually — some live wallpapers will drain your battery faster, and others will barely affect it at all.
Why Do Some Android Live Wallpapers Drain Your Battery More Than Others?
The effect of a live wallpaper on your battery depends on how well-made the wallpaper is. Some of the third-party ones (often free on the Play store) will drain your battery really fast, because your phone's processor is constantly at work to keep the wallpaper doing its thing.
If the wallpaper is particularly badly-designed, it might even continue to drain your battery when the screen is off, or when you're using an app.
Some live wallpapers aren't so bad, though. It really does depend on which you use.
How To Pick A Good Live Wallpaper
If you're downloading from the Play store, I'd suggest reading the reviews first. If people are saying that the wallpaper drains battery, take their word for it — it probably does!
There are quite a few Android forums on the internet, and they usually have a lot of information on these sorts of things.
Another idea is to just test your phone with and without a live wallpaper.
You could also just use them on special occasions — I had a snowy live wallpaper at Christmas a couple of years ago, and although it drained the battery a bit, I was only running it for about a week.
Ultimately, if you have to charge your phone battery every night anyway, you may as well just get a live wallpaper if you want one!