Very interesting paper on the economics of prostitution by Steven Levitt and Sudhir Venkatesh.
The transaction-level data we collected suggests that street prostitution yields an average wage of $27 per hour. Given the relatively limited hours that active prostitutes work, this generates less than $20,000 annually for a women working year round in prostitution. While the wage of a prostitute is four times greater than the non-prostitution earnings these women report (approximately $7 per hour), there are tremendous risks associated with life as a prostitute. According to our estimates, a woman working as a prostitute would expect an annual average of a dozen incidents of violence and 300 instances of unprotected sex.
The authors also noted that a prostitute was "more likely to have sex with a police officer than to get officially arrested by one". (via marginal revolution)