San Diego Seawater Desalination San Diego San Diego is building one of the first sea water desalination plants in California and it is also going to be one of the largest.  The estimated cost is $1 billion dollars, but the return on investment (ROI) is calculated to be well worth the expenditure.   "The Water Purchase Agreement sets the price of water from the Carlsbad Desalination Plant at $2,131 to $2,367 per acre-foot in 2016, depending on how much is purchased annually. The first 48,000 acre-feet of water purchased each year will pay for the fixed costs of the project and the variable costs of water production. The Water Authority has the option to purchase an additional 8,000 acre-feet per year at a lower rate that reflects only the variable costs of incremental water production." - Index of ... Seawater Desalination Seawater Desalination - expanded