that sounds very reasonable

My point exactly. Desalinating IS more expensive than taking water out of the river that flows by your town. But if you don't have a natural river or lake nearby (like all of coastal Southern California) it costs about the same to transport the water hundreds of miles as it does to desalinate it locally.

This is why all schemes involving transporting water even greater distances are just plain stupid: Even if the water is free, the transportation cost kills the deal.

On the other hand, water cost can never rise much above what it is now in Southern California because the cost hits an upper limit that is the desalination cost. The Sierra snow pack and the Colorado River could dry up completely and we could just desalinate 100% of our water for about the same cost. Instead of coastal Southern California drying to a parched desert, it will bloom. Can't say the same for western states far from the ocean.