An average-sized pol, expect to pay anywhere from $25,000 to $55,000. If you want one of those gigantic pools like you see in the movies, be prepared to part with over $60,000 at least. You will be getting a lot for that kind of money, but you need to be absolutely certain this is what you want, before going out and spending your savings.
An in-ground pool is also harder to winterize than an above-ground one. The entire pool will need to be cleaned, as will all its parts (pumps, feeders, etc.) You also need to tend to the underground pipe system, draining them, drying them, and then plugging them, to ensure that nothing seeps into the pool while it's sealed up for the winter.
Also do not forget that an in-ground pool is part of the ground, so make absolutely sure this is something you want. If you decide, down the line, that you don't want the pool anymore, you're unfortunately stuck with it.