The two combat stars are set to face each other on 26 August in a fight which many believe McGregor has no chance of winning, mainly due to the fact that he has no professional boxing experience while he is also facing arguably the greatest boxer of all time.
The Irishman was once a 10/1 betting underdog to beat Mayweather on many betting lines when the fight was announced.
However, he is now less than a 4/1 underdog (7/2 to be precise) according to a Bovada press release as per MMAFighting.
McGregor, known as "Mystic Mac" for his accurate predictions on how his fights will end, notably claimed he would finish off Mayweather within four rounds.
Given the media coverage, worldwide attention and fan-following of McGregor, it seems many are now taking a punt on the "Notorious" to emerge victorious. However, that would not be good news for oddsmakers.
Kevin Bradley, Sports Book Manager of Bovada, was quoted as saying that oddsmakers "will need Mayweather [to win] huge", and that an early knockout win for McGregor "would be a potential disaster".
Other betting lines have also opened such as McGregor being 11/1 to throw a kick during the fight as well as whether the whole event would break the pay-per-view record set by Mayweather's fight with Manny Pacquiao.
As for the odds of a "Money" win, the same press release has a Mayweather victory at odds of 1/5.