Re: What percentage of fights are rigged?
It depends on your definition of "rigged." Overall, I think boxing greatly parallels the NBA.
If you mean odds tipped somewhat or even heavily in the direction of one fighter, then many. Just like home teams in the NBA get extra calls and free throws which leads to extra points and more wins, home crowds often influence judging. The other fighter can overcome the obstacles, but is facing an uphill battle.
If you mean blatantly pre-determined, then I would say a few per year. Just like when the NBA suddenly changes officials during a playoff series and the team down 2-0 wins, or when they hand the Lakers 27 free throw attempts in the last 10 minutes of a game 6.
It would be far too difficult to pre-determine the outcome of an MLB game, slightly less so for the NFL. The NBA is likely as rigged, if not more rigged, than boxing.