"he had for years been known as a professional beggar" [TWIS]
Readers of this blog are undoubtedly aware that Neville St. Clair, in "The Man with the Twisted Lip" left his respectable job as a journalist to take up the life of a professional beggar under the nom de travail Hugh Boone.
In the June 27 installment of The Straight Dope, the question was posed to the research staff: "How much money do beggars make?"
"Data! Data! Data! I cannot make bricks without clay."
While there are little empirical data to go on, the author leads with the example from the Canon and even goes so far as to quote Leslie Klinger, BSI, editor of The New Annotated Sherlock Holmes.