Price ranges vary depending on the material of the rack, heat source, and whether or not tubs are included. Usually they're heated by flexwatt, I think most common is a 4 inch or three inch strip which runs along the back of each level of the rack.

Heres a picture: http://www.boaphileplastics.com/rhin...xwatt_Rear.jpg

Usually this is enough to provide the proper temp gradient your snake needs; you may run into problems if your enclosure is in too cold of a room. In that scenario you would need to do something to raise your cool side temp.