Although costs have been rising, water is still an inexpensive resource in most American towns. An average cost is less than $5 per 1000 gallons. Filling a swimming pool, in most areas, costs less than $100.You’ll need to add some replacement water during the year, to replace splash out and backwash water.ost trucks hold 5000 gallons or so, and can make repeated trips to the hydrant until your pool is full. Water truck companies usually charge by the load, and a complete fill of the pool can be expensive, as much as $50 per 1000 gallons.