On Thursday, we wrote about how Groupon CEO Andrew Mason is only making a salary of $575 this year. But don’t reach for the hankies just yet. The 29-year-old has already made a killing on his two-and-a-half-year-old company.

According to Groupon’s S-1 filing, Mason has already cashed in almost $28 million worth of stock–just under $18 million after the company got its Series F financing in April 2010, and $10 million when the company completed its Series G financing earlier this year.

Doing even better are Mason’s cofounders. Two LLC’s owned by Eric Lefkofsky, Mason’s former boss who convinced him to drop out of graduate school to pursue the business that would become Groupon, and his wife have cashed in almost $380 million in stock. And an LLC owned by the wife and children of cofounder Brad Keywell has cashed in almost $157 million in stock. (For complete details, see p. 112 of the filing.)

All three still own significant amounts of the company. Mason has 7.7 percent. Lefkofsky, who financed the original operation, has 21.6 percent. And Keywell has 6.9 percent.

And it’s not necessarily unusual for shareholders to cash in stock before a company has gone public. “Sometimes, we advise our execs or companies we’re looking to invest in to ‘take some chips off the table’ so that they can continue to make bold, risky bets for their startup and not be worried about 100% of their net worth being tied up in the future of the company,” a staffer at an investment firm writes on Quora, addressing this very question.

“Groupon made some incredibly bold bets by expanding internationally so quickly,” she continues. “They made perhaps an even bolder bet by turning down Google’s offer [purportedly to buy Groupon for $6 billion]. I can imagine encouraging the founders/employees to take some liquidity so they can plunge ahead.”

Nevertheless, the fact that the company used much of its Series F and Series G money to cash in shares has raised eyebrows. Of the $135 million the company raised in its Series F, it kept only $15 million for operating costs and used the rest to redeem stock. Of the $946 million it raised in the Series G, it kept $136.2 million for operating costs and used $809.8 million to redeem stock, including some belonging to early investors like Accel and New Enterprise Associates.