What do you want to do in the garage and for how long? What temp do you want to maintain? Is this to work in the garage or just keep your car warm? No heater is economical but that also depends upon how rich or poor you are. Get your electric heater or a ceiling mounted gas heater or a wood burner or a space heater and because of the extra heat loss through the doors, oversize it for normal btu's needed to heat the space if insulated. Even a wood plank sided shack (or a tent for that matter(ever been in the Army)) will concentrate heat inside but will loose it faster than an insulated one and will require the heater to provide more heat more often to allow for the heat loss.
I've worked outside with a space heater blowing on us and it helps to keep a little warmth. At 10 below it was economical to me. They use them on the sidelines at pro football games at Buffalo, etc. A clean burning propane space heater with a blower is nice to have and you can move it around and they don't take long to heat up. I have a 250,000 btu kerosene space heater, which is what most construction crews use in an unheated house while they are working on it. It just smells like kerosene fumes all the time is the bad thing.