Heating a garage shouldn't be that costly depending how often you're out in the garage and how high the heat setting is and what type of heating fuel you use.
I have a 2 car garage and I sometimes go out into the garage and work on my lawn tractor and vehicles during the cold winter months or just tinker around out in the garage and I have a ceiling mounted natural gas forced air heater and it stays nice and warm in the garage.
It costs me maybe $45.00 to $60.00 per month to heat the garage up depending on how often I'm out in the garage.