Water cliche alert.
The former mayor of Phoenix wants to see desalination plants lining the Sea of Cortez to supply future water needs in Northern Mexico and Southern Arizona because he says "water is going to become, if it’s not already, more valuable than gold or oil.” Apparently the economy is humming along so nicely in Phoenix they believe they are going to be the next Saudi Arabia. Why Saudi Arabia? Because that is the only place I know of relying heavily on desalinated water for municipal and industrial (M&I) needs (it may have something to do with the cheap cost of power in that region). If you're talking about using desal to supply irrigation water (which is where most of the water is used within areas that border the Sea of Cortez) you are in similarly sparse company. Israel and Spain are about the only places currently using desalinated water for irrigation on anything resembling a large scale and much of that use involves treatment of brackish groundwater rather than seawater. Those types of waters have a much lower dissolved mineral content than seawater, so production costs are also much lower, but it still isn't cheap water.
The part of this equation I find especially puzzling is the fact that if you are attempting to grow crops with desalinated water (even from brackish waters) the cost of the produced water is going to be significantly higher than the cost of the next likely water supply. In order to make farming work with such high input costs you need to become much more efficient in your water use and probably grow higher value crops. Growing higher value crops would justify the expense of installing highly efficient irrigation systems and paying higher costs for irrigation water. But if it's economical to switch to low-water-use irrigation technology and you can significantly reduce the amount of water being used by agriculture do you really need to build desal plants or are you freeing up enough water that had been going to agriculture by improving irrigation efficiencies that you can meet other projected needs? And if you have to install the on-farm efficiency to justify the use of desalinated water can you then save the expense of building and operating desal plants and instead put that money into greater water use efficiency - effectively creating the necessary new supplies simply through conservation? I don't think anyone will argue that building and operating desal plants (and the additional energy supply needed for those plants) is comparable cost-wise to constructing and maintaining highly efficient irrigation systems.
Look at some numbers: If you have 1 million acre-feet of water going to irrigated farming and they are currently paying $30/ac-ft for their water, that is an input to the farming operations of $30 million per year. If you want to replace 20% of that water with desalinated water that costs $400/ac-ft you have additional input costs of ($400-$30) x 200,000 ac-ft = $7.4 million. And those would be annual cost increases. It would make good business sense to spend more than $7.4 million in one year to improve on-farm irrigation efficiency in order to save a similar amount of water because once you apply a discount rate to the investment, the cost per acre-foot of additional water will clearly be lower than the cost of desalinated water. This is admittedly a simple example that is missing a lot of key detail, but I would challenge anyone to show me that the cost per acre-foot of desal water would be competitive with the similar cost per acre-foot of conserved water.
Desal may be a sensible option at some point far in the future, but there are still cheaper options out there that can add significant water to existing supplies.