Got me one of these a few years ago- after doing a lot of research:
Amazon.com: Mr. Heater Big Maxx 75,000 BTU Natural Gas Garage Unit Heater #MHU75NG: Home & Garden
That's $494 for 75,000 btus- a most excellent price. I paid a bit over $500 when I bought it. And you can never have enough BTUs.
I couldn't be happier- my garage can go from 35 to 70 in about ten minutes (a bit bigger than yours with high ceilings). And the gas costs one heck of a lot less than electricity. You can install it yourself- biggest PITA is cutting the hole for the flue, the rest is easy.
The only thing I would recommend is throw away the thermostat that comes with it and get one from the Hardware store.