To clear things up, what zone are you?
I think we live in similar climates. I have 1 1500watt hot water heater and 1 5000watt heater for the air, plus about 1gallon of water/ square foot of greenhouse. My greenhouse is 800 square feet, 2 sheets of plastic with a little inflation fan. I can keep it above 30F, but its a stretch during the coldest part of the winter. My heating bill is $200-$300.
Don Schnell has an 8x12. I think he has 3-4 1500 watt electric heaters under his benches. He keeps it at 40F, but I believe he brings his neps inside for the winter. He's in zone 5.