The kind of bespoke construction in Wright's buildings couldn't be built today at an order of magnitude higher price, even considering inflation. A side effect of mass produced standard construction materials has been custom ones becoming astronomically expensive due to the skilled labor to build them having been replaced with mass production.

I suspect projects like fallingwater have siting considerations that wouldn't allow it to be built at all anywhere in the US... isn't it built basically on top of a WOTUS?

It'd be in litigation forever; which is why nobody with the means would try to build something like that today. Even if they could afford the construction, they can't afford the time in court.

Larry Ellison owns a replica Japanese daimyo mansion in Woodside, two mansions on Bellevue Avenue in Newport, and 98% of the island of Lanai...but none of those structures there are (AFAIK) atop a permanent watercourse.