If either fighter tests positive it seems there will be more than 5 million dollars to pay. No fight (with all the money that is worth) and a four year ban. Which at their respective ages would surely equal an end to their careers

"Travis Tygart, the chief executive of the United States Anti-Doping Agency, said a positive test would kill the May 2 fight, where the total purse likely will be more than $200 million. It also would subject the fighter who tested positive to a potentially career-ending ban of four years from competition.
“If there’s a positive test prior to the fight, the penalty for the fighter who violates it is going to be a lot higher than $5 million,” Tygart said Friday.
Both fighters signed agreements for the USADA to oversee drug testing for the May 2 fight that will be boxing’s richest ever. Mayweather is expected to make at least $120 million and Pacquiao $80 million for the much anticipated bout"

Official says penalty for positive test by Mayweather or Pacquiao is severe