Half a cent per gallon is 7,727 times MORE per gallon than a Los Angeles resident typically pays if they manage to stay in Tier 1 pricing all year. For facts concerning Los Angeles water rates see: https://www.ladwp.com/ladwp/fa... .
You are orders of magnitude off in understanding pricing in the water commodity market. Not that RO can't be done, just about every golf course and condo Cabo San Lucas BSC MX is watered via reverse osmosis. However, the valuations of each of those condos is in the millions per 1,000 sq ft so the investment makes sense for the developers. When the average home price in California picks up a couple more digits, RO will make perfect sense.