The cost of the propane is negligible. One $5 tank will likely last you at least 3 game days. I don't pull my trigger very often, so it's hard for me to estimate how other people would last.
However, the nature of gas guns, is such that you won't be dumping 120 rounds in a 30 second fire fight. So lasting 3 or 4 games is partially because you won't be shooting as much as you would with an AEG.