I dont have a real rule of thumb put typically it will cost about 3500.00 if you contract it out and it depends on the City you are in some cities require a permit and complete haul of others dont mind if you collapse the walls and back fill with the old concrete then top with dirt. Just make sure you bust holes in the bottom if you keep the bottom part of pool so that the area does not hold water.
If you require additional info please call me at 682-597-6814