I have a true to life example of heating costs Although my temps are quite a bit warmer than you! This is on my 8x16x10 g/h with 4mm twin wall poly. The heater maintains 60f but so far has only been used at night as the days have no problem warming up (and requiring the cooler to keep it down around 85f) I filled a 100lb propane tank on 11/13/09 to the tune of $51.35 (2.20/gal @22 gals) I was in the g/h this morning before work packing an order when the heater went out (already had the other tank in my truck to fill today ) Anyway... That means that tank lasted me 20 days only being used to heat at night at 60f when my nights have been somewhere in the high 30s,40s and low 50s. That's about $2.57 a day. Now, although- we're starting to get cooler... Soon it will be time to take the shade cloth off to aid in day time warming.
I agree with all the stuff Tony has said, heating is fairly easy but not cheap. Cooling and keeping humidity up is the trick (especially for me!).