By Keith Idec

After listening to Conor McGregor mock him Tuesday, Floyd Mayweather Jr. reached into this backpack and pulled out his $100 million check from the Manny Pacquiao fight two years ago.

McGregor’s quip drew laughter from many of the thousands of fans who filled Staples Center for the first official Mayweather-McGregor press conference in Los Angeles. Even though he reportedly owes $22 million to the IRS in back taxes, Mayweather made sure to get the last laugh.

“You right,” Mayweather told a seated McGregor. “I’m the IRS and I’m gonna tax your ass!”

Mayweather went on to belittle the UFC superstar’s earning power, which has increased dramatically now that Mayweather has agreed to fight him in a 12-round, 154-pound boxing match August 26 at T-Mobile Arena in Las Vegas (Showtime Pay-Per-View; $99.95 in HD).

“Yeah, we knew I was the ‘A’ side,” Mayweather said. “That’s why it say Mayweather first. That’s why this bitch had to come to the boxing ring.”

The unbeaten boxing legend also addressed McGregor’s cracks about Mayweather wearing “a track suit” to the first stop on a four-city, three-country, four-day press tour that’ll continue Wednesday in Toronto.

“He look good for a seven-figure fighter,” Mayweather said. “He look good for an eight-figure fighter. But motherf***er, I’m a nine-figure fighter. And y’all know what? This bitch made $3 million his last fight. But we know that’s training camp money for me.”

The 40-year-old Mayweather (49-0, 26 KOs) continued taunting McGregor (21-3, 18 KOs in MMA), who initiated the trash talking because he spoke first.

“And guess what? I’m an old man,” said Mayweather, who’s slightly more than a 5-1 favorite. “I’m not even the same man I was 20 years ago. I’m not the same fighter I was 10 years ago. I’m not the same fighter I was five years ago. I’m not the same fighter I was two years ago. But I got enough to beat you.”

McGregor predicted Tuesday that he will knock out Mayweather within four rounds in the first official boxing match of his career. Mayweather made a similar knockout guarantee.

“I’m guaranteeing you this – you going out on your face, or you going out on your back,” said Mayweather, who hasn’t scored a knockout since he stopped Victor Ortiz in the fourth round of their September 2011 fight in Las Vegas. “Which way you wanna go? That’s right, sit quiet, you little bitch.”

Keith Idec is a senior writer/columnist for BoxingScene.com. He can be reached on Twitter @Idecboxing.