If Earth were smoothed so it didn't any changes in elevation the earth surface would be under 3000' of ocean. But humans want fresh water as compared to saltwater- in order to drink or grow plants.

The Wall Street Journal is whining about how we running out resources:
http://online.wsj.com/article/SB120613138379155707.html?mod=hpp_us_whats_news&mod=WSJBlog

Far as I can tell it's pretty stupid article, but anyways one of main beefs is lack of water. In article they say, quote:
"This troubles some economists who used to be skeptical of the premise of "The Limits to Growth." As a young economist 30 years ago, Joseph Stiglitz said flatly: "There is not a persuasive case to be made that we face a problem from the exhaustion of our resources in the short or medium run." "
And directly follows this with:
"Today, the Nobel laureate is concerned that oil is underpriced relative to the cost of carbon emissions, and that key resources such as water are often provided free. "In the absence of market signals, there's no way the market will solve these problems," he says. "How do we make people who have gotten something for free start paying for it? That's really hard. If our patterns of living, our patterns of consumption are imitated, as others are striving to do, the world probably is not viable." "

As though this Stiglitz has changed his mind- instead of the obvious of Stiglitz stating there is a market problem which is that governments are underpricing water- and thereby not allowing markets to address the problem. No where is the article does it mention the typical price of water. But do say silly things like: "Despite advances, desalination remains costly and energy intensive."

So, different article:
Desalinated water would cost $2,023 to $2,996 per acre-foot. Presently, it costs the district about $1,000 per acre-foot for water. An acre-foot can supply about three single-family homes for a year.
http://www.marinij.com/marin/ci_7381128
Another article:
"Now, the cost to desalt seawater is pegged around $800 per acre-foot, while MWD water costs $431 per acre-foot. The drop in desalted prices is due largely to advances in the filtration process, in which membranes used to filter out the salt have become more efficient and less energy is required to pressurize the seawater.

In one closely watched project, the Tampa Bay Water agency is building a seawater desalination plant that when operational early next year will produce 25 million gallons of drinking water a day (enough for the needs of about 100,000 residents) and cost about $680 per acre-foot of water."

Other source:
A promising method to desalinate seawater is the "reverse osmosis" method. Right now, the high cost of desalinization has kept it from being used more often, as it can cost over $1,000 per acre-foot to desalinate seawater as compared to about $200 per acre-foot for water from normal supply sources. Desalinization technology is improving and costs are falling, though, and Tampa Bay, FL is currently desalinizing water at a cost of only $650 per acre foot.
...
Here are our parameters for saline water:
Fresh water - Less than 1,000 ppm
Slightly saline water - From 1,000 ppm to 3,000 ppm
Moderatly saline water - From 3,000 ppm to 10,000 ppm
Highly saline water - From 10,000 ppm to 35,000 ppm

By the way, ocean water contains about 35,000 ppm of salt."
http://ga.water.usgs.gov/edu/drinkseawater.html

Maybe one of the differences between +$2000 per acre foot and $650 per acre foot is how much salt is removed- I don't know.

More:
"The efficiency of desalination is between 15-50%; in other words, 15-50 gallons of potable water are produced for every 100 gallons of seawater. The remaining water consists of brine and dissolved solids. The energy required to produce this potable water from seawater by reverse osmosis is approximately 2,500-12,000 kilowatt hours per acre-foot, depending on the quantity of salt in the intake water. (California Coastal Commission 1993). This high energy requirement for facility operations combined with high capital investment results in water costs substantially exceeding costs for traditional water sources. For example, the cost of producing potable water through desalination ranges between $1,300-$2,200 per acre-foot, depending on its salt content and necessary pretreatment. In contrast, the Metropolitan Water District of Southern California water costs range between approximately $195 per acre-foot for imported water and up to $300 per acre-foot for water delivered by the district to some of their customers."
http://www.resources.ca.gov/ocean/97Agenda/Chap5Desal.html
And same article:
"Recent technical improvements in desalination technology, such as more effective cleaning compounds and membranes which require less pressure, have improved its competitive position in comparison to traditional water treatment and supplies. When located with a power facility, desalination can produce potable water from seawater for less than $1,000 per acre-foot and potentially as little as $500 per acre-foot (California State Assembly 1990)."