How much does it cost to flush a urinal?

The cost of flushing a urinal (or a toilet) would depend on that city or area's charges for water usage and for sewage, which are usually separate charges. It would also depend on how many times these are flushed. For example, a 60-year old man going to urinate 6 times a day would spend far less in water and sewage than a building housing grades 9 to 12 with hundreds or thousands of students. Therefore, it is impossible to answer your question without additional information. However, the account holder could call your local water company to ask the charge for 1 gallon of water plus sewage costs, then find out how much water is used to flush a specific urinal, and you could figure out the cost for only that urinal.