Do you have a figure for price per gallon? It might be interesting to do a cost benefit analysis between desalination and the water pipeline from Canada mentioned elsewhere in the comments.
The best I have is from [1], which estimates 50 million gallons a day at an ongoing operating cost of around $50 million per year. You'd have to pick a timeframe to also amortize the cost of the plant itself and add that in.
[1] https://www.wsj.com/articles/SB10001424127887324049504578545...