The simple answer is, the ...

The simple answer is, the cost of desalination, regardless of  method is directly proportional to the TDS of the feedwater.   There are many other factors involved however such as the cost of delivering the feedwater to the desal system.   Consider the desal system as box that receives water on one side from any proposed source, processes and delivers a suitable product our of the other side of the box.  Plug in all the basic parameters and that should give your a good start.   Then of course there are plenty of qualified persons who have typically 30 to 50 years of experience in all aspects of desalination who make their living providing the very best solution.

A major consideration for tapping brackish sources is the fact that they are finite and can have repercussions when depleted whereas seawater is infinite.