Californians living in urban areas use an average of about 124 gallons of water per day.
California has 38M residents of whom about 3M (8%) are illegal aliens.
3M X 124g/day = 372,000,000 gallons of water / day.
San Diego County will be buying $3 billion over 30 years for only about 7% of the county’s water needs from a new desalination plant. San Diego County has a population of 3.1M. http://www.nytimes.com/2013/03/01/business/energy-environmen...
It follows that, given today's technology and assuming no additional illegal immigration, to produce enough water for California's 3M illegal aliens over 30 years will cost about $103B or $3.4B/year or $1,133 annually per illegal alien.