because there was a man nick named 50 cent and he was a gangsta/drug dealer that everybody looked up to him and he died so curtis "50 cent" Jackson took that name to show respect
He took the name from a criminal named "Kelvin 50 Martin (July 24, 1964- October 24, 1987)" Kelvin got the name as he would rob anyone regardless of how much money they had so he would rob someone...