“if the cost of operating all year when the mikva may not be well used, is defrayed by the increased revenue from Erev Yom kippur, then that is part of the cost.”
Of course it is. If you want the mikveh to be open and available to you for the twice a year you use it, they have to charge you money so that can cover their annual operating expenses. It happens to be, the costs increase on the 2 days a year most people want to use it because they require additional resources.
again, if $10-$15 is too steep, go to the beach.