I heat mine with a Reznor 75K natural gas furnace. The shop is 980 sq. ft. I keep it at 50 when I'm not out there and only bring it up to 60-65 when I use it. A 60K might do the job more efficiently, but if you're lowering the temp way down, (like I do) the 75K can bring the temp back up quick.
My gas cost has only been about 100 per month in Jan/Feb. That's using it about 2hrs a night and weekends.
The shop was spray foam insulated so it's pretty tight, except for the two 8x10 doors.