My garage is 25x21 and I have a used 85K BTU forced air furnace I got for $150 and it came with a plenum and a "B" vent chimney. I got it from a heating contractor that keeps the old ones he replaces and checks them out for safe use. When people add on to their homes they often need to upgrade to a larger furnace. I keep it at about 50 F most of the time and when I work in there I can bring it up to 70 F in about 5 minutes. I figure it adds about $15 per month to my NG bill from before I had this extra furnace. It takes about a 2x3 footprint and I have doubled up on the filters as I do a lot of woodwork in my garage, so I need to keep up with the dust. As well, I shut it off when I am painting something until the paint has flashed. I think that is the cheapest way to heat your garage / shop.
