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!
really your best bet is to think of what devices ur gona use, lights, heatmats etc and work out the wattage, then find out how much ur electric company charges.
a hole load of vivs wont cost anymore to run than say..an elecric fire however as those things are generally 2000 watts and i reckon cost about 20p an hour.
i had this idea turn a shelf into a rep vivs has this been dione before and another question what size viv would a dwarf monitor want or what is the smallest monitor and what size viv need-adult monitor that is
If he doesnt have the space maybe he should wait until he does?! What will happen if you break up? Will you be stuck with the monitor because he doesnt have room to house it? Will it have to be rehomed? Obviously you arent planning on breaking up but these things happen.