Nobody said that that is the proper way to pronounce 50 but It is the proper way to pronounce it for the rapper/mogul.
I'm not rewriting the dictionary, it is what it is.
Born and raised in New York, Brooklyn that is..
You walk into a bank and say "can I have 200 dollars in Fifties?"
Then 50 Cent walks in you greet him by saying What's up Fit tee?