Pretty good article in CNN Money.
http://money.cnn.com/2014/03/12/news...omy/index.html
A study released Wednesday by the Urban Institute's Justice Policy Center analyzes the size and structure of the underground commercial sex economies in eight major cities: San Diego, Seattle, Dallas, Denver, Washington, D.C., Miami, Atlanta and Kansas City, Missouri
No idea how factual this $500,000 study was, probably exaggerated a little like everything else.