From the straight dope:

http://www.straightdope.com/columns/read/2255/how-much-money-do-beggars-make

"I bet this won't surprise you, but estimates vary. As Michael S. Scott, the director of the Center for Problem-Oriented Policing, says in his online article "Panhandling": Estimates vary from a couple of dollars (U.S.) a day on the low end, to $20 to $50 a day in the mid-range, to about $300 a day on the high end. Women, especially those who have children with them, and panhandlers who appear to be disabled tend to receive more money. For this reason, some panhandlers pretend to be disabled and/or war veterans. Others use pets as a means of evoking sympathy from passersby. Panhandlers' regular donors can account for up to half their receipts. In a study of Toronto panhandlers conducted by Robit Bose and Stephen Hwang, panhandlers reported a median monthly income equivalent to US $190-$200. "

I really like Tyler Cowen's position on the issue. He basically said when in a third world country that receives lots of first world tourists, there usually are a huge number of beggars. Many of these people could get work, but they would make less than they can as skilled beggars. Instead of rewarding this non-productive work, he gives money to poor people who are cooking, offering to work as guides, etc. This way he incentivizes doing productive work, while still giving money to the poor.