At the moment my shed (used to be a small garage) is more than hot enough, so I just use heat mats on stats like you would if the reps were in the house. The reps will be moving by the time winter comes, but last winter we installed a tubular heater (usually used to heat greenhouses) to keep the ambient temperatures up.
This doesn't seem to use an awful lot of electricity, I havent seen the bills shoot up anyway.
The best way to keep bills down is to insulate the shed well, using celotex or a similar product (dont forget to insulate the roof!) If your garage is attached to the house it may already have some insulation - my parents garage does, it also has a radiator!
I can get you some pics if you want, but there isnt much to see!