The economics of prostitution
The Economist has gathered statistics that I didn't know exist to determine the value of prostitution in various cities, and also considering other variables. The illegality of the trade in most places would make the gathering of statistics difficult, it would seem. I suspect that many of these attributes correspond to variables that draw attention on dating sites, at least the attributes that can be gleaned from looking at a photo (to determine body type, race, length of hair).