How does having a pool affect your monthly water bill?

Once you've filled your pool, and taken that one time hit, there will be some routine water usage to replace water lost by evaporation. It is possible to get enough rain to replace it, so the cost might be zero. If you get no rain, and it's hot and sunny, you might loose up to a foot of water in a month. You will also loose some water in periodic backwashing of your filter. So you could be looking at 10-20% of your pool volume per month in makeup water. I'm no expert, just reporting my own experience.