Don't worry about RAM consumption. Android is designed to use as much memory as possible, which allows for more efficient task switching and lower battery consumption. If you have any battery/memory optimizing apps that have a RAM cleaner, you would be better off uninstalling it.
As for live wallpaper use, anything animated is going to use more power than displaying a static image. How much power depends on the specific LWP. A simple animation wouldn't require much. One that reacts to motion or uses your camera (I.e. making the home screen "transparent") is going to require more power. B. Diddy is right for his specific use case, but this is one of those variable situations where you'll have to just try it for yourself and results may differ.