Ok well... I bought a 55,000BTU propane powered mini torpedo. I also have an electric quartz radiant heater. The propane mini torpedo was only $139 at The Tractor Store. I bought an extra 20# cylinder as well. The torpedo will run for about 8 hours on high from a 20# tank. If I warm up the garage with the torpedo first, and then shut it down and use the electric radiant heater to keep the chill off while working, this might work out.
If I need a boost from the torpedo from time to time, then I'll have to consider what operation I'm doing and how much dust will be in the air. Might have to make better use of my dust control. I don't always run it.
I have those interlocking cushioned mats in my work area and I REALLY can't spend more than a couple hours on my feet at a time anyways. Trying to work out there in winter is going to force me into some better habits as far as planning and cleaning. If I kept the propane heater running on high, AND if I cut the run time estimate to 6 hours instead of 8, then it'll cost me about $2 an hour to run it. (I can fill a 20# cylinder for $12).
The other morning when I went out to go to work, the outside air temp was 22 degrees. The garage inside was 34. Even in the dead of winter, snow MELTS off my van after I pull it in and the water on the floor from the melt has never frozen in the 13 years we've been here. However... water in a water bottle HAS frozen. So I'm guessing the concrete might not cold soak but the air gets below freezing at least sometimes.
When I put on the addition for the shop, I might plan on electric heat. I know there's 220 out there because the guy that lived here before used to restore old farm tractors out there and had an arc welder out there. I'll have an electrician take a look at the electric system from the mains to the sub panel and let a pro tell me if it could handle the load. If I just keep the shop at 40 or 50 degrees when I'm not out there then it MIGHT not be terrible on the electric bill. Might also be a good enough reason to make the walls from 2x6 instead of 2x4 and get more insulation in there. The mice will love it!
I wonder if I'd be better off with the sprayed on, expanding foam stuff.... I wonder if mice don't like that.
ANYWAYS.... I'm rambling as usual. For now.... and with a careful eye towards dust, I think I can pop the temp up with the torpedo and maintain some level of working comfort with the electric quartz heater. I won't always be using dust-making power tools. I'd like to be able to go out there and sit and work on carving, or do things with hand tools. I also have plans around here somewhere for a "dust detector". It works kinda like a smoke detector and just measures stuff suspended in the air... like smoke... or dust. If that alarm goes off I could make sure the propane heater ain't running.