"I was in London and I wanted to exchange pounds. So I gave him $500 and he basically gave me back £200. I was like 'What?! How does this work?' This is how the Billionaire thing started out. I started humming it on the streets. Travie came along later and put his verses on it and then it hit the charts." Bruno Mars on the inspiration for Billionaire, his song with Travie McCoy.