It’s in the article excerpt that 20 percent of the country gets its water from it; the $500 million will obviously be amortized over the lifetime of the plant, and the operating costs of a typical large RO plant used to somewhat exceed $1 per cubic meter. The article states that this is cheaper. The figure for annual operating cost could be estimated by the number of people to be served and the per capita water use. In most societies, domestic, personal water use is under 10 percent of total use, most is used for other things, such as irrigation, and commercial and industrial uses (including food-related processes like canning, bottling, cleaning).
Based on a recent population estimate 6.2 million, one fifth of the population paid $404 a head for the construction, about $13 a year for 30 years; the per capita water use in Israel (2007 figure) is 281.9 cubic meters, but that’s all uses (including agricultural — Israel didn’t invent hydroponics and trickle irrigation, but is state of the art in both areas; used to be a net exporter, mostly to Europe, now a net importer I believe).