This was Mark Cuban's 6-figure splurge after he became a millionaire

Circa 1990, Mark Cuban was dining with friends. He'd just sold his first business, MicroSolutions, to CompuServe for a reported $6 million.

So Cuban's pals had question: "They asked me what I wanted to do with the money I just had gotten," Cuban tells CNBC Make It.

It was a question Cuban had thought a lot about. Since founding MicroSolutions seven years earlier at age 25, he'd been solely dedicated to work and had gone all those years without a vacation, according to his blog.

"I gave up a lot, personally, early on to try to accomplish my goals," Cuban said on "The Thrive Global Podcast," "knowing that if I ever reach the levels of success even close to where I ended up, I was just gonna have fun."

By the time of the dinner, Cuban had decided it was finally time to do just that.

So Cuban pitched an idea to his pals: "I told them I wanted a lifetime pass on [American Airlines] so I could go anywhere anytime," Cuban tells CNBC Make It.

In 1981, American Airlines introduced the lifetime unlimited AAirpass, a ticket to fly anywhere as often as you wanted — for a steep fee. The passes cost $250,000 when they were introduced, according to the Los Angeles Times, and buyers could shell out another $150,000 to add a companion pass. In 1990 the pass cost $600,000, including a companion ticket. (For comparison, that amount in January 1990 had same buying power as $1,175,293.56 in March 2018, adjusting for inflation.) In 1993, the price increased to $1.01 million, and by 1994, American stopped selling the unlimited passes — perhaps because one owner of the pass flew over 40 million miles, according to the Los Angeles Times.

But Cuban had decided globetrotting would be a fun way to spend his windfall.

"We were at an old school steakhouse that had phones at the tables," Cuban explains to CNBC Make It. "So I used the phone to call [American Airlines] and asked them if they sold life time passes.

"All my friends were drunk when I bought it," he adds.

The paperwork showed up a few days later. While Cuban says he doesn't remember exactly how much he paid nearly 30 years ago, he figured it to be a smart investment at the time.

"I guessed/calculated that my predicted cost per mile would be 12 cents," he says. "That was a good deal to me. So I bought it."

Today, only a few dozen people still own an unlimited pass, according to an American Airlines spokesman, and they are still valid.

Of course, Cuban is now a multibillionaire with his own private plane, so who knows if he's flying commercial anymore.

But at the time, the splurge was one of the "most fun, best business decisions I ever made," Cuban tweeted Monday. He used the AAirpass "to go party like a madman," he says on the "Thrive Global Podcast."

"I don't know the number," he tells CNBC Make It of the countless trips he took. "I went everywhere."

Don't miss: Mark Cuban says quitting this one common bad habit made him more successful

Like this story? Like CNBC Make It on Facebook!

Disclosure: CNBC owns the exclusive off-network cable rights to "Shark Tank."