How much is a heat pad for a crested gecko?

A heating pad is not necessary for a bearded dragon. Heating should come from overhead in the form of UVA and UVB lighting. The idea behind heating pads is a good one, but they have been known to get too hot and burn/fry your bearded dragon to death. However, the overhead lighting will heat rocks constantly exposed to the lights to some extent, but never hotter than the temperature than the light already produces, so they shouldn't prove to be a danger.