If Floyd Mayweather's expected rout of Andre Berto on Sept. 12 at the MGM Grand in Las Vegas truly was his final fight, then Mayweather will have gone out with a whimper, not with a bang.

Mayweather, boxing's clear-cut pound-for-pound king and the highest-paid athlete in the world, retained his welterweight world titles against Berto and announced his retirement afterward, as expected. But although he closed his career with a perfect record (49-0, 26 KOs), sports fans largely yawned at his supposed finale.

According to multiple industry sources, the Mayweather-Berto Showtime PPV card was a box-office bomb, selling 550,000 subscriptions, at best. It could have done as few as 400,000, one of the sources said.

"Being generous, it might hit 550,000 buys," one source said.

Floyd Mayweather's Sept. 12 bout with Andre Berto drew a disappointing 550,000 pay-per-view buys, at best -- a weak total compared to the whopping 4.6 million from his May 2 showdown with Manny Pacquiao. Joe Camporeale/USA TODAY Sports

That would make the fight by far the worst-performing bout of Mayweather's now-expired six-fight deal signed in 2013. It is his weakest pay-per-view performance since he was an HBO PPV rookie in 2005 and 2006 for fights against Arturo Gatti and Zab Judah.

Those bouts were profitable because they did not come with the giant expense of Mayweather-Berto, which cost $36 million in purses for the main event alone -- $32 million for Mayweather and $4 million for Berto -- not to mention millions more in expenses for the rest of the card and the overall promotion.

At around $70 a pop, a little lower for standard definition and a little higher for high definition, if the fight lands at 550,000 buys, then it generated around $38 million. But a little less than half of the gross goes to the cable and satellite providers, likely leaving the promotion with less than $20 million from pay-per-view sales.

Even with other revenue streams from the live gate, foreign television and sponsors, the fight likely will be a money loser. The live gate was Mayweather's weakest in years, with the crowd of 13,395 not close to being a sellout.

Stephen Espinoza, the executive vice president and general manager of Showtime Sports, declined to discuss the Mayweather-Berto pay-per-view tally, but he did not dispute the 550,000 figure.

There are a lot of reasons the fight sold poorly, including wide public dissatisfaction with his record-shattering win against Manny Pacquiao on May 2, for which Mayweather earned around $250 million. The pay-per-view sales record for that fight has been amended from the originally announced 4.4 million buys to 4.6 million. In addition, few gave Berto, a massive underdog, a chance to be competitive, and few believe that it was truly Mayweather's last fight.

"I think Floyd is a victim of his own success," Espinoza said. "No matter what we did following that massive May 2 event, it was almost guaranteed to be viewed as a letdown. Was Mayweather-Berto the biggest Mayweather fight ever? No. But we got four very entertaining fights on the pay-per-view card and a historic night in that Floyd announced his retirement. All in all, we're very happy with the event as a whole."

As for the low sales, Espinoza offered a supposed cause.

"We know that a large chunk of the audience watches Floyd to see him lose," he said. "And for those people, the best chance of somebody beating Floyd was Manny Pacquiao. So once Floyd beat Manny, and beat him definitively, a lot of the intrigue was gone. If Manny couldn't beat him, nobody would beat him. So they didn't buy the fight. I also think we were suffering a little bit of a hangover over from May 2."

Espinoza acknowledged that the selection of a better and more dangerous opponent, such as Amir Khan or Keith Thurman, might have come close to the 1 million sales mark Mayweather usually generates.

Even though the Berto fight was a bomb, Espinoza said Showtime/CBS was very happy with the overall Mayweather contract, which generated around $800 million from more than 10 million pay-per-view buys for the six fights. Two of the fights, against Canelo Alvarez in September 2013 and then Pacquiao, broke the record for all-time revenue for a fight.

"If you had told me back at the time we signed the deal we would get two record-setting fights, that Floyd would fight two fights a year for the term of the contract and we'd get fights with two guys most people thought Floyd would never fight -- Pacquiao and Canelo -- then I would have said this is an absolutely astounding deal.

"In short, the Mayweather deal has far exceeded our expectations financially. In 2½ years, we had six pay-per-view events that generated over 10 million pay-per-view buys and $800 million in gross revenue from domestic pay-per-view. No matter how you slice it, this deal is a huge win for Showtime, CBS and Floyd."

Espinoza said that at some point he will talk to Mayweather, a five-division champion, about the prospect of returning to the ring.

"We'll let some time pass before we have that conversation," Espinoza said. "He's been absolutely consistent in saying this is his last fight. He seemed very content and at peace with that after the fight. We show some of that in the locker room after the fight on the 'All Access' epilogue episode on Saturday night. It was like a farewell. But there will definitely be some texts and phone conversations and attempts to take his temperature on an ongoing basis.

"Personally, I don't see a return to boxing for him in the near future. He's been doing it for 19 years, and this countdown to his retirement has been in his head since he signed this deal. He felt a duty to play out all six fights in the timetable he promised he would, and now, he feels he has nothing left to prove. Where he'll be in a year, nobody knows. But he seems pretty content with his decision."

Espinoza said at some point he would talk to Mayweather about whether he might be interested in a role as a commentator. He said Showtime will continue to televise cards put on by Mayweather Promotions as well.

The Mayweather-Berto replay premieres on Showtime on Saturday at 9 p.m. ET/PT, followed by the debut of the "All Access" epilogue episode.