Good question, and really I use same rule of thumb as apprasiers do, if you can use other "true" comps without factoring in the pool, do so. If not, depending on the pool and please understand it can be a huge difference between a plastic stand up pool and an in ground, salt water deep black pearl "infinity" pool. In general, $10K for standard size plaster or low end pebble tec up to $20K for larger with water features.
Are you using a an agent or Realtor? They can help big time with all these. As a home owner with a pool, the cost is so much more than people realize. Add at least $50 a month in summer for electric use on the pool pump, if you pay someone else for chemicals, $50 a month but you still need to clean it yourself or that is more. Plus chemicals and etc, in other words I would much, much rather have a community pool and pay a HOA. Course, everyone is different but that is my opinion on it.
Best of luck.