15/month? how many days/hours you running it. I have a similar unit its a 5,000 watt unit... works great but you will be better off starting with a natural gas unit.
Look at your power bill and see what you are paying per KWH and then use a calculator to see what your usage will be per hour based on the watts of an electric heater. I pay about .06 per kwh. If I ran mine 8 hours a day it would cost around 72/month.
i guess if you ran it for 50 hours or less per month you could do it for 15/month. Keep in mind it takes an hour or two to get the chill out depending on your garage size.