I have a 300k BTU / hour heater. My gas company charges per therm currently $0.3996. 1 therm = 100k BTU. The cost to run the heater one hour is 3 therms * $0.3996 = $1.20. I have a 20,000 gallon pool. One BTU raises one pound of water by 1 Fahrenheit. There are 8.34 pounds in a gallon, so it takes 8.34 BTU to raise a gallon of water by 1 F. A 20,000 gallon pool requires 166,800 BTU or 1.67 therms. Given my price of gas from above, it costs 1.67 therms * $0.3996 = $0.67 to raise my pool by 1 F.

I heated the pool from 65 F to about 85 F a couple of days ago for a total of $13.35. This was much less than I expected.