Well, that would definitely be the biggest fight in history. Because in MMA, Mayweather has actually got a shot - there are some example of high level MMA fighters losing suddenly to high level specialists with little cross training (Aoki getting KOd by that kickboxer, Mercer KOing Sylvia, arguably Lesnar very early on in his career beating Herring and Mir etc). He wouldn't be favored to win, but it is waaaaaaay more interesting than the boxing match, and that alone would hype it to 6 million PPVs probably.