Several answers to this question, and they appeared in earlier comment – I am not being mechadesh anything.
1. Ask anyone who built a mikva in their shul. The expenses for construction are not simple, nor are the details that relate to the halachic requirements. Until the actual construction gets done, there have already been numerous consultations. That is a capital expense. In reality, anyone who attempts to build a mikva is most likely to be doing so on a donation – hefty one. Yes, there are cost overruns, too.
2. The maintenance expenses of a mikva are huge, often a bigger undertaking than the original investment in construction. Water, boiler, showers, soap, towels, basic housekeeping and maintenance for the operation, all cost. These expenses must be borne by the users. Most mikvaos that are used daily have “chodesh gelt”, where there is a flat fee paid monthly. This cuts down on the cost for the user, and helps the mikva only towards breaking even for their expenses. Having the money up front is useful, as this would in any other business.
3. There is a perspective change needed here. Instead of seeing the prices rise with Erev Shabbos, Erev Yom Tov, and Erev Rosh Hashanah and Yom Kippur, look at the rest of the year as discounted.