Now, book operator Jay Rood is working hard to balance the action.
"Hopefully by this evening we'll have taken a million-dollar bet on Mayweather," Rood said Thursday. "We're trying to get that hooked up now."
Big bets and big fights are nothing new in this gambling city. But the richest fight ever is also drawing more money at the betting window than any fight before it.
"The handle on the Strip will be astronomical," Rood said. "It will easily become the biggest fight in Nevada books. I think it could fall somewhere between $60-80 million."
Pacquiao fans have led the way, betting their fighter early and often in most of the city's legal sports books. The influx of money on the underdog has caused odds to drop to less than 2-1 in most books, though some larger Mayweather bets have kept them from falling even more.
Bookies say they expect the betting to rival that of some Super Bowls, fueling a weekend at the sports books like no other as bettors line up to be the fight, the Kentucky Derby and the NBA and NHL playoffs.
"It's amazing. I've never seen so much action so early on a fight," said Johnny Avello, who runs the book at the Wynn resort. "Going back two and a half months this has generated business every day and we haven't even gotten to fight day yet."
Oddsmakers say the betting public is infatuated with Pacquiao, partly because they like the Filipino fighter and partly because he offers more value as a plus-170 underdog. At the William Hill chain in Las Vegas, four out of every five tickets have been on Pacquiao to win the fight, and the ratio is even more lopsided at some local casinos.
A lot of the bigger tickets, though, are on Mayweather, including the bettor who was negotiating for the $1 million bet at the MGM Grand, just down the hall from the fight arena.
A day earlier, Rood said the book took over $1 million in total bets on the fight, with 60 per cent of them on Pacquiao.
"It's great value for either fighter," he said. "We haven't seen two-way action like this in a long time on a big fight."
At the South Point hotel off the Las Vegas Strip, oddsmaker Jimmy Vaccaro said he had never seen betting on a fight like this in a career that stretches over nearly four decades.
Vaccaro said Pacquiao bets outnumber Mayweather wagers 5-1 at his book, with the fight tracking to do Super Bowl numbers.
"Strip away the props (proposition bets) from the Super Bowl and Pacquiao versus Mayweather is going against Seattle and New England," he said. "I think for the state it will be about $70 million just on this fight."
Jay Kornegay of the Westgate LV Superbook said he will take more large wagers — both five and six figures — on the fight than he did on the Super Bowl.
"Most of the tickets are on Pacquiao but the bigger money will come in on Mayweather," Kornegay said. "The line itself is very attractive not only to the average Joe but also for the deep pockets out there not afraid to lay two dollars on Mayweather because he's not normally that short of a price."
One of the more popular bets at most books has been on the fight ending up a draw, largely because conspiracy theorists believe it would lead to a lucrative rematch.
A draw for a typical fight might start at 30-1, but Rood said he put this fight out at 22-1 because he expected more money on a draw. But the volume of money has been so heavy on a draw that the line is now 6-1.
"The draw is the land mine for us right now," Rood said. "It should be higher but our risk on it now is considerable."