There is quite a bit of variation in central Florida, depending on the utility serving a local area and extent to which saltwater intrusion along coastal areas requires use of desalinization or long distance transmission. In Orlando, we sit over the Floridan Aquifer, an enormous freshwater "river" slowly flowing thru the pervious rock substrate deep below. It is one of the purest sources of freshwater in North America. ..... So, the water is relatively cheap compared to other areas of the state, but, nonetheless, there is a sliding scale of charges to discourage heavy water use. The first 3,000 gallons per month is $0.801 per thousand gals. The next 12,000 gals is $1.071 per thousand. The next 15,000 gals is $1.881 per thousand. Usage over 30,000 gals per month is $2.931 per thousand. So, 60,000 gals would cost approx $160. The larger charge is the sewer tax on water discharged into the sewer system.It is about double the cost of the water, so that $160 would become more nearly $480. But, no sewer tax applies if the water is used to irrigate and separately metered on an irrigation line. It costs a bit to acquire a separate irrigation meter, but the cost is quickly recouped.
Bottom line: You could do a 10% daily water change on a 20,000 pond for about $5 per day, if the waste water is used to irrigate the garden.
Or, a 50% weekly water change on a 5,000 gal pond for less than $15 per month.