https://www.watereuse.org/sites/default/files/u8/Power_consumption_white_paper.pdf
At the scale of 50 million gallons of water per day, desalination using reverse osmosis is about 10.4 kilowatt hours per 1000 gallons.
At our electric rate of 12.6 cents/kWh (which is much lower than California), that's $1.31 per 1000 gallons. That's acceptable for residential usage: we used 4,000 gallons last month, and that's an additional $5.24.
But what about agriculture irrigation? I wasn't able to find numbers specific for California in a quick search, but I was surprised at the result.
http://pubs.usgs.gov/circ/1405/
Public-water supply was #3 in 2010, at 14% of freshwater use.
Agricultural irrigation was #2 in 2010, at 38% of freshwater use.
The #1 use of freshwater, at 38% of freshwater and 45% of all water (including saline), was thermoelectric power generation
So one has to ask: how much additional freshwater will be needed by power-generation to desalinate a gallon of sea water?
I found it here:
http://pubs.usgs.gov/circ/2004/circ1268/htdocs/table04.html
For California, in 2000 they used 15,200 million gallons per day (MGD).
Public Supply was 2,800 MGD, or 18.42%.
Irrigation was 11,600 MGD, or 76.32%.