Jim DeLaney, Austintown, Ohio
I'm in NE Ohio - similar climate to yours.
My shop is about the same size as yours, but with 10 foot ceilings, and insulated about the same as yours.
I have a 35,000 Btu radiant heater (natural gas) mounted high on an end wall, with an 'auto-setback' thermostat. It keeps the shop at 65 all day, and 55 at night. If I need it warmer (like when finishing) , the heater will bring it from 65° to 72° in about 20~30 minutes.
I've figured it costs me about $20.00 per month to keep the shop (and me) warm.