(...) an immutable law of labor: when there are a lot of people willing and able to do a job, that job generally doesn't pay well. This is one of four meaningful factors that determine a wage. The others are the specialized skills a job requires, the unpleasentness of a job, and the demand for services the job fulfills.
The delicate balance between these factors helps explain why, for instance, the typical prostitute earns more than the typical architect. It may seem as though she should. The architect would appear to be more skilled (as the word is usually defined) and better educated (again, as usually defined). But little girls don't go up dreaming of becoming prostitutes, so the supply of potencial prostitutes is relatively small. Their skills, while not necessarily "specialized", are practiced in a very specialized context. The job is unpleasent and forbidding in at least to significant ways: the likelihood of violence and the lost opportunity of having a stable family life. As for demand? Let's just say that an architect is more likely to hire a prostitute than vice versa.
Steven D. Levitt & Stephen J. Dubner, Freakonomics, ed. Allen Lane 2005