I forgot to read my meter before heating, but from 4pm on Saturday until today I used 83m^3 of gas. I turned the heater off at around 4 on Monday. So I missed the initial heat up, but to keep the water at 85F for two days I used, at most, 83m^3 of gas.
Looking at my bill a little more closely it looks like my all in variable gas cost is actually about $0.35/m^3 if I include distribution, transmission and the carbon tax, in addition to the direct cost of gas. So to keep the pool heated for two days cost about $29, less if you account for the fact that some of the gas I consumed would have been for cooking or heating water. It probably cost an equal amount to heat the water up in the first place, so I am probably looking at about $50 for three days of swimming. Today was another very warm day and the water is still at 80F. So still comfortably swimmable, if I wanted to do so.