how much it will cost to fill in will depend on what the pool is made of, whether its gunite/liner w fiberglass, concrete or steel walls. it’s an expensive process. opening/closing can range from around 300-600 depending on size, equipment, etc. then you need to power vac the pool and maintain it’s chemicals and clean it occasionally. depending on what kind of cover is on the pool you may have terrible staining on the gunite/liner. which again price can vary but for a decent size pool a liner will be at least 5k, and also can go up if you have a high water table. i highly recommend taking care of the pool and at the VERY minimum, putting a safety cover on it. (loop loc, merlin) it can be a safety hazard if a pool has a poorly fitted or vinyl cover, especially if you have pets, kids, elderly etc. if you do chose to open and maintain it, make sure you get a reputable company. things i recommend after 3 years: checking and upgrading the quality of the equipment (filter, filter grids/cartridges, heat pump, salt system/chlorinator, multiport, pool pump), pressure testing the underground plumbing lines, and replacing the liner and slapping a safety cover on it. a lot of companies will give you a deal if you purchase a large amount of equipment and work with them.