How do you know I’m not in Brooklyn?
Because you paid $15 to use the mikvah.
I think they charge more on erev Yom Tov because people are going to come anyhow, but charging more during the year will cause there to be fewer people to come.
It’s simple economics.
Most (almost all?) mikvaos are not self funding, they operate partially on donations, so I don’t see where there’s any tayna.