DALLAS -- Chalk up another remarkable feat for Michael Phelps: somehow making swimming trading cards popular.
An autographed 2004 trading card of the record-shattering Olympian was trading for as much as $500 on Thursday, just two weeks after industry experts say the collectible could be easily had for $25.
"It's pretty good," a grinning Phelps said Friday when informed of the price in Beijing, where he won his sixth gold medal.
The trading card is only a part of Phelps' long-stated effort to elevate interest in swimming to more than once every four years.
"I heard the [TV] ratings are the highest back home for people watching the Olympics and watching swimming," he said. "My goal of raising the bar in the sport of swimming is coming along nicely, but it's still going to take time for me to really get it to where I want it to be."
The market value could rise to $750 to $1,000 if Phelps breaks Mark Spitz's record of seven gold medals at a single Olympics this weekend, said Tracy Hackler, an associate publisher with Beckett Media LLP, a Dallas-based memorabilia company.
"It's unlike anything we've seen in the trading card category," said Hackler, whose company is an industry leader in collectibles pricing.
Arlington, Texas-based Donruss produced about 300 of the autographed Phelps cards in 2004 and about 5,000 "common" cards not bearing the American swimmer's autograph, Donruss spokesman Scott Prusha said.
The non-autographed cards have enjoyed a spike in market value too, going from $2.50 to about $20, Hackler said.
Phelps has won six gold medals, setting six world records in the process, at the Beijing Olympics. He picked up his 10th and 11th career gold medals Wednesday to become the winningest Olympian of all time.
The card was part of a special "Fans of the Game" insert by Donruss that paired celebrities with their favorite teams. Phelps' card includes him next to the Baltimore skyline and a pennant of his favorite NFL team, the Ravens.
Others cards in the set included actors John Travolta, James Gandolfini and Charlie Sheen. But none has gotten the response of Phelps' card.
Hackler said Beckett listed the card at $150 on Wednesday and $350 on Thursday, when he said it was trading online for upward of $500.
"We're kind of in uncharted waters here," Hackler said. "No pun intended."