I am considering a diesel/ kerosene heater, but not sure on its running costs
I only use it a few hours at a time to blow heat where I'm working. My garage is uninsulated also, so obviously a waste of energy trying to heat it.
Only thing is it may not be great for a woodworking workshop. Fumes from it not noticeable in my garage, but it's not for enclosed use
Can't say I was cold in there last week though. Overalls, latex gloves and a decent jacket helps a lot. The gloves make a huge difference, even though you wouldn't think it.