i bought one of these. paid about $150 CDN for it. 80 000 BTU
http://www.mrheater.com/productdetails_extended.asp?catid=45&id=25
my garage is a 2.5 car garage with NO insulation and NO drywall. one day when it was -31F outside i fired it up on high just to see how warm i could get it, i ended up getting it up to like 35F and i could work in a sweater.
the trick with propane is ventalation if you wanna live lol. i had a electric heater blowing air in from the window to get some fresh air in. what i ussually do is run it on HIGH for about 15 mins till its warm enough then i run it on low while i'm working, once i feel the cold coming back i crank it on high for another 10 mins and turn it back down. the propane has an odor when burnt but you get used to it really quick
BTW where i live winter is -30F for alot of the winter