Garage Heat?

As you can see there are allot of different options : I have a finished off 24 x 40 garage with R19 in the walls & 12" blown in the ceiling - Natural gas is far less expensive in my area (Chicago) - I have an older gas furnace from a motel - properly vented with just a box plenum on top to direct the heat - we've finally had a winter this year and my highest combined heating bill for my modest house and the garage (with the thermostat set no lower than 50 degrees) has averaged $97.00 from November threw Feb and I crank it up to 65 when I'm out there putt-sing around. Can't wait until I can see my lawn again... Good Luck - Reindeer