The vast majority of bets placed on this weekend's bout have been for McGregor, the Irish mixed martial arts superstar.
The vast majority of bets placed on this weekend's bout have been for McGregor, the Irish mixed martial arts superstar who is facing Mayweather in what will be his first ever boxing contest.
With Mayweather a 2/9 favourite, punters in Las Vegas have gravitated towards McGregor, whose odds have dropped to 5-1 with some bookmakers.
Nick Bogdanovich, William Hill US's director of trading, told AFP that bets on McGregor were outstripping wagers on Mayweather by a ratio of 17 to 1.
"It's been insane," Bogdanovich said. "We've got a massive decision on our hands because the McGregor money just keeps pouring in. It's been fun."
"It's bigger than any fight we've ever booked in the state of Nevada. It's almost like a Super Bowl."
Bogdanovich said the fight had appealed to casual fans tempted by a big payday if McGregor somehow manages to pull off what would be a monumental upset.
"The people that are betting is revealing -- it's your meter-maid, it's your butcher, it's your garbage man, your every day Joe is involved in this fight," Bogdanovich said.
"They're willing to bet a little to win a lot. They think that if they put up their $100 (84 euros) they get $500 back and that appeals to them. We knew it would appeal to them, we just didn't think it would appeal to that many. We underestimated exactly how many people would be backing Mr McGregor."
Bookmakers face seven-figure losses if McGregor defeats Mayweather, with one estimate predicting $48 million losses.
Bogdanovich said the oddsmakers would absorb the hit.
"It's no big deal. If we lose, we pay, and we open up for business the next day," he said, confident that the tale of the tape still favoured the bookies."
"We're in a pretty good spot I think. Mayweather is probably the greatest pound for pound fighter ever, he's 49-0 and he's fighting a guy who's 0-0 and who's never been in a 12-round boxing match. The challenge is so big."
"You're asking someone's first boxing match to be against a legend. That's a tall order."
One of the most popular novelty bets is for McGregor to lose by disqualification, he added.
"A lot of people believe that McGregor will get frustrated and eventually snap and do something Tyson-like," Bogdanovich said, referring to former heavyweight Mike Tyson's infamous 1997 chomp on Evander Holyfield's ear.
Some of the biggest individual wagers include a bettor who placed $300,000 on Mayweather to win. The bettor would net $60,000 if the American is victorious, according to William Hill.
Another anonymous bettor placed an $880,000 stake on Mayweather at one casino, a wager that stands to recoup $160,000 if successful, according to US reports.