There is a good reason why electric garage heaters are so popular - they are really easy to install. You just plug them into the main supply and you have heat.
They can also be pretty cheap in terms of original outlay - for a couple of hundred dollars you can get a quality product that will keep five hundred square feet warm. On top of that they are really easy to move around.
More often that not, these are not particularly energy efficient space heaters. When it says they are efficient it is often referring to the speed with which it will heat up an area - how many British Thermal Units per Hour and so on. It is probably not making great claims for the low running cost.
Sometimes this does not matter. Depends on how much you use the garage and other things - the savings could be pretty negligible between an electric unit and those propane space heaters which are often cheaper to run as well as pretty impressive heaters.
When it comes to this kind of decision you may want to use some other criteria. It may be more important that you buy USA made electric space heaters than a few bucks of saving (Of course US made products often offer the best specs).