The shop is kept at a constant 50 degrees, when I go out I bump it to 65 or so, wait 30 minutes, and then I can take off my heavy outer clothes, and run around with a T shirt on. I don't have AIR, ounce in a while I wish I did, but heat is a must here. Trying to do woodwork in the 20's or less, just doesn't work. At 40 cast iron takes on a different life, where a touch can send your entire body into shock. Kept 50 or better, and it's just cast iron. 

Our house AC bit the dust a few years ago. It was cheaper to replace both the furnace, and AC coil, rather than just put in a new coil. so we did, and the guys moved the old furnace out to the shop. I built a stand to mount it up into the air to their specs. It houses 4 sets of filters, and the furnace itself stays dust free, runs fine, and just has an outlet pointed toward the shop direction. I thought without ducts it would be weird. It heats like it's supposed to, and soon the entire 32 x 48 shop is toasty. If/when it bites the dust, the plan is to mount one of those propane shop furnaces in it's place. All of the work for propane is already done, to hang and vent a burner unit would be 2 hours of shop time, and a few bux for parts plus the cost of whatever unit they used. I own a 500 gallon tank, this year it was $284 to top it to 80%. I think it's easily worth that to be comfy. I have less than a grand in getting the old furnace back and running, they did all the heavy lifting, all I did was build the pedestal it sits up on. That was about 1/16 of my total plywood scrap pile, and some screws. I run it from September through April, kept at 50. Some days it hardly runs, others it runs almost all day. I might run out of fuel if I kept it at 65 or better all the time, not sure, but I have no plans to keep those mice that comfortable. When I'm back there, they scatter and leave, so my Wife, and I, are the only ones taking advantage of 65 degree temps back there.