A new mikvah should of course be designed on environmental principles. I am amazed each time I think about the first mikvah I built, back in the early 80's, that we didn't consider the amount of clean water taken, or the amount of energy used to heat that water, every time it is used.
Just a couple of years later, I was thoroughly aware and concerned about the changes we were causing to the climate. A new mikvah should have a small spa pump and heater and use chlorine tablets to keep the water clean, rather than use new water each time!
These things don't come cheap! But we can take steps to minimise cost by good design - and sensible 'halachic' decisions. For example, if you can keep the water in the mikvah as described above, then you only need one store tank, not the two I used previously: as long as the mikvah is full, you can empty the store tank, clean it and let it refill from subsequent rain - the mikvah water, once valid, remains so. This depends on accepting the principle of the pump/spa system. According to halacha, a pump is problematic as it counts as a 'chamber' and therefore water coming out of it is 'drawn'. Yet where you have very orthodox mikva'ot (eg Chabad Hampstead Garden Suburb), one for men and one for women, the men's one at least utilises this system - basically, I believe, because a) they are not so serious about men (as there is no longer a Temple, so they don't have to reach a level of complete purity), so it is only 'custom' anyway, and b) the traditional approach tends to always be more lenient about men than women (probably because it is decided by men)! From our point of view, I'd suggest that the water returning from the pump passes along the earthenware or concrete channel for 1 mt, thereby 'revalidating' it, using the legal fiction that we have relied on up to now (and which the orthodox also use, albeit only in a 'belt and braces' situation!).
The main cost issue (aside from the cost of the land and building) is that the two tanks need to be 'part of the building', which really means cast in concrete, and tiled (because a flat tile cannot be a 'container'), but really that is just a small built-in indoor spa, and a swimming pool installer should be able to do it at reasonable cost.
