For the last couple of years I have used a 15,000 btu blue flame
ventless propane heater I purchased off of ebay. I have it sitting on
a milk crate in the garage, but it can be hung on the wall as the back
of it is always quite cool.

My garage is insulated, but has many air gaps around the garage door
and good infiltration at the light switch I haven't closed up in the
drywall, some places in the ceiling with no drywall, but insulation.

When the heater is on, you can stick your hand up into the 8' ceilings
and feel the hot air up there. Since there is substantial storage
above my garage, you an also feel the heat rolling in from the heater
across the garage when you open the access door.

You need to insulate those walls, that made a BIG difference in my
little garage (15x22).

Propane ran me about $.50 an hour (propane in the $2.20 range), the
same BTU's in electricity here would be less 1/2 of that. (7.2 per
kWh)

Currently in the process of putting in a radiant floor heated shop.
Planning on electrical water heater for that heatsource. Gas used to
he half 6 years ago to use, but the price increased have made it MUCH
closer, add that to the extra cost of the gas water heater, and you
don't get a payback in the life of the water heater.