No mention of the cost so it's probably not cheaper than traditional sources. It's just cheaper than the incredibly expensive existing plants.

What do you mean by "cost"?

The article does mention the cost to the Israeli water authority:

   Sorek will profitably sell water to the Israeli
   water authority for 58 U.S. cents per cubic meter
   (1,000 liters, or about what one person in Israel
   uses per week)

One has to believe that the cost to produce the water is directly tied to the sales price. I'm sure that this plant couldn't have been built without some sort of long term guarantees as to price, and I'm sure no rational government would have agreed to buy the water without the price paid being tied directly to cost of production plus some reasonable rate of return.