I don't think cheap and safe really go together as far as heaters go.
Cost of running the heater is an issue as the previous poster pointed out. If your garage is unheated which I assume it is, since you pointed out how cold it gets there your heaters are going to be on a lot. If you can heat your tank with 1000 watts and your heaters are on all the time to maintain the temperature. It's going to cost approximately 60 dollars a month to heat if your electricity is only .08 kWh.
I'd recommend buying a point of use tankless water heater and a stainless pump and connect to a ranco temperature controller to maintain your temperature. It'd probably cost 500 for the setup, but be cheaper to use.