Because I’m bored- Googling “cost to desalinate water” gives $2k per family of five, and googling “population of California” gives roughly a population of 40 million. 40mil/5 * 2k == 16 billion (if I’m counting zeroes right.)

So it’s in the ballpark of a percent of California’s total GDP (which is roughly 2 trillion, IIRC) for non-industrial uses. (But industry, including agriculture, is responsible for that GDP- how easily a “dry” Californian economy could support desalination can’t be inferred from these numbers!)