My dad insulated his single car garage and didn't have much money left over to spend on a heater so he bought a 230v 18,000BTU portable contstruction heater. These past few days have been about the coldest it ever gets here and that heater gets the garage up to 50-55F so it's not bad at all, not super hot but very workable in a sweater and an extra pair of pants. He said some guru calculated the cost of that thing to be about 23 cents an hour. And you're only heating it while you're working in there so I don't think he'd ever spend more than 15 bucks a month heating that garage.