I stumbled across an interesting academic paper titled "A Theory of Prostitution" which investigates how this low-skill, labor intensive job brings high income rewards. The paper investigates the role of marriage, the economics of marriage and prostitution and makes postulations in accordance with the statistics. They also play with the (interesting?) notion that wives and whores are actually economic goods: women sell, men buy! To quote the paper: "a prostitute sells non-reproductive sex, which we will call "commercial sex", while a wife sells reproductive sex (i.e. sex plus children)."

Grab the PDF file from here. It is an interesting read.