“Assuming the original construction was already paid for, is there any other major upkeep if no one is using it?”
Who paid for the original construction? The twice a year mikva goers? If they did, then let them set the price for those 2x a year. More than like there is a mortgage or some other type of loan that financed the construction. The bank likes to get paid every month, not once a year after yom Kippur when the twice a year crowd showed up and paid $10.