That’s a really difficult question to answer. Will you be burning propane or do you have natural gas at your house? The vented southern burner is rated at 25,000 BTU, so it will burn just over 1 liter of propane per hour running non-stop. Of course, how much it actually burns will depend on the outside temperature, thermostat setting, and how well insulated the greenhouse is. Also, since the venter southern burner has an open combustion chamber, you have to have outside air constantly entering the greenhouse, so you can’t make the greenhouse really “tight.”
For what its worth, with my current setup using the direct-vent Empire (which I really like), I have a 250 gallon (946 liter) propane tank that is filled two or three times each winter Last winter was cold—three fills. This winter, I have only had one delivery so far. The tank is usually filled when it is down to about 25% full and is filled to 80-85%.
Very roughly, I think my fuel costs are $50-100/month averaged across the year. I guess that’s high, but it’s less than a lot of people pay for their cable TV package or other entertainment.