I don't think the price you pay has everything to do with the cost of the water, but the distribution, servicing, cleaning of human-consumable quality water. The one price I do know for DeSalinated (among the most expensive) was $0.45/m^3 or 1000 liters (Hyflux 10 year commit, industrial quantity). The problem, of course, being that you need to be near the ocean.