In the grand theft auto games, where does the wheelie money come from?

In the grand theft auto games, you are instantly awarded cash for wheelies and stoppies. From where?In the grand theft auto games, where does the wheelie money come from?
You never find out, I've always assumed a hobo sees you do a Wheelie and gives you his cash, however, in GTA4 you don't get money for doing wheelies.