“For a good blow job, a man will do just about anything. What can I do with that knowledge? I have no idea.”
How Tech Tools Transformed New York's Sex Trade is an article by Sudhir Venkatesh demonstrating the results from a survey of 290 sex workers in New York about financial matters. Venkatesh and Stephen Leavitt did a similar study in Chicago, posted to Metafilter previously. More information about Venkatesh's research on sex work on his website.
Steven Leavitt and Sudhir Venkatesh, of Freakonomics fame, investigate prostitution. According to the working paper [pdf], prostitutes in Chicago are more likely to sleep with the police than get arrested by them.