It's expensive. But it's slowly becoming more economical as water importation costs are rising.

Ranges for capital costs for reverse osmosis are $700 to $900 per cubic meter to $1,300 to $2,200 per cubic meter. Ranges in reported O&M costs for reverse osmosis are from $0.45 to $0.92 per cubic meter. 60 percent of the O&M costs are energy costs. 3 to 3.5 kWh per cubic meet of water is used at a production rate of approximately 40 percent of the salty water being turned to fresh water. Somebody smarter than me can figure out how much it would cost to build a facility to treat enough water for LA.