The biggest plant in the US, near San Diego (Carlsbad), cost $1B to build and produces 200,000 m^3 per day. Israel's biggest desalination plant, Sorek, cost $400MM and produces over 600,000 m^3 per day, so 40% the cost and 300% the water. So I assume a large part of the cost of the Carlsbad plant is just all the usual reasons everything is insanely expensive to build in California.

https://en.wikipedia.org/wiki/Claude_%22Bud%22_Lewis_Carlsba...

https://www.water-technology.net/projects/sorek-desalination...

The Carlsbad plant is in Carlsbad, CA. If you look at the neighborhood it's in, the houses are 1,200-2,000 square feet and sell for $2-$5 million. Everything is expensive.