I spent under $100/mo in the summer because I did all the upkeep myself and just bought supplies (salt pool was easy and awesome).  However I had to replace my pool pump after 3 years ($1200) and I had a leak.... Good thing it was under warranty, but those leaks can be very expensive.  Also I had cracks develop on the aprons/deck area that were unsightly.  

  Bottom line:  With kids and in a hot location where you can use it more than 6 months out of the year pools are awesome.  However I would never do it again......