What do you get when you combine the richest prize fight in the history of United States boxing with the United States’ most lucrative annual horse race? For Las Vegas sportsbooks, the answer is a betting event second only to the Super Bowl in terms of volume and possibly larger in terms of overall generated revenue.

Bets have been rolling in for weeks ahead of Floyd Mayweather’s once-in-a-generation bout against longtime rival Manny Pacquiao, with countless more expected before they meet in the ring at the MGM Grand Garden Arena Saturday night. The Kentucky Derby, already considered horse-racing’s biggest annual gambling draw, should benefit from the droves of seasoned wagers who will flood booked-out Vegas casinos to bet the fight. It’s a perfect storm, one several times larger than the average sports Saturday and with the power to make or break a sportsbook’s annual earnings.

“I’m expecting probably one of the top five days in Las Vegas history as far as sports gambling goes,” said Tony Miller, director of the race and sports book at the Golden Nugget casino in Las Vegas, Nevada.

The Super Bowl and the Kentucky Derby are traditionally considered the largest sports betting days of the year, each drawing tens of millions of dollars in wagers. The Super Bowl alone generated a record $119 million in betting in 2014, according to Forbes. But the addition of the Mayweather-Pacquiao fight provides a new wrinkle to this year's Kentucky Derby wagering and should place May 2 among the Super Bowl-dominated list of Vegas’ top betting events. By any estimation, it is the most lucrative fight in boxing history.

Pre-fight projections placed expected revenue at an unprecedented $400 million, ESPN reported. The fight’s live gate will reportedly be more than three-and-a-half times larger than boxing’s previous record, and secondary market tickets have reached obscene heights. In late February, when the fight was first announced, experts estimated the statewide sportsbook take at anywhere from $30 million to significantly higher. As of now, “significantly higher” seems to be the safe bet.

“It’s going to be an unprecedented weekend for gambling, and not only sports gambling,” said Alan Silver, a noted casino industry expert and assistant professor at Ohio University. “I think the table games and everything are going to have huge drops.

Hotel rooms throughout Las Vegas have been booked for weeks, with prices in the hundreds of dollars for a single night. While just about 15,000 individuals will receive the privilege of watching the fight in person, tens of thousands more paid $150 to watch the fight via closed-circuit feed at MGM-owned properties throughout Las Vegas, the Guardian reported. Untold others traveled to Las Vegas simply to be in the vicinity – and to place bets on the day’s events.

Mayweather, who owns an undefeated 47-0 record as a professional, is the odds-on favorite to win the superfight. Vegas-based sportsbook Bovada currently lists Mayweather at minus-210, meaning gamblers would have to bet $210 to win $100. Pacquiao, meanwhile, is listed at plus-170, meaning gamblers can bet $100 to win $170.

Gamblers can also place wagers on proposition, or “prop bets,” such as whether or not the fight will end in the knockout or the specific round in which the fight will end. These bets account for a full third of the total money wagered on the fight, said Kevin Bradley, Bovada’s sportsbook manager.

The “smart money” is being placed on Mayweather, as seasoned sports bettors look to cash in on the expected outcome. But most of the bets at both the Bovada and Golden Nugget’s sportsbooks have been placed on Pacquiao, with whom the payout is larger. At this point, sportsbooks are hoping for more Mayweather money and dreading the possibility of a Pacquiao upset, particularly if it fulfills prop bets that call for a knockout.

“If Pacquiao wins in an early round by knockout, it’s a complete disaster and we’re not making that money back,” Bradley said. “There’s not going to be another fight like this.”

But it’s hard not to get excited about May 2’s slate of action, regardless of where the money will ultimately go. Sports days of this magnitude come along once every few decades.

“I just think the combination of the two [events] is going to make this one of the biggest Saturdays we’ve seen in Vegas in probably 20 or 30 years,” Miller said.