A similar project has been on construction in Europe
https://de.m.wikipedia.org/wiki/Nant_de_Drance
Cost of $2B. I had the chance to visit the caverne where the turbines will be, really amazing engineering!
Those pump storages provide the very expensive peek electricity all over Europe, needed for the spikes in the morning and noon.
Nevada probably didn't need it back then, Hoover Dam was built for flood protection, and since then the levels are too low to work efficiently as pump storage. But since peek energy is in high demand they think it over.