What's Phelps' Medal Worth?

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How much is Michael Phelps' 19th medal worth? Anywhere from a few thousand to $50,000 to $1 million, depending whom you ask. A collector, a jeweler and an ad agency looking to hire Phelps to endorse a product all have different answers.

What might a collector pay for Phelps's gold? "If it came on the market tomorrow," says expert Jim Greensfelder, referring to the swimmer's latest, history-making medal, "maybe $50,000, assuming all the documentation came with it."

Greensfelder is co-author of "Olympic Medals: A Reference Guide," which describes every Olympic medal ever made—its dimensions, weight, and the mint that produced it.

There's no prohibition, he says, on an athlete's selling his or her medals. Olympic medals turn up for sale on eBay or are bought and sold through dealers specializing in Olympic memorabilia—for example, he says, David Feldman in Switzerland. Olympic gold medals in general, he says, bring a price of about $10,000. The average price for all medals (gold, silver and bronze combined) from any Olympics is around $3,000.

Considered purely as jewelry, Phelps's medal isn't worth much: Its gold content is negligible. The only gold medals that were really gold—solid gold—were small ones made for the 1904, '08 and '12 Olympics, according to Greensfelder.

Experts agree a medal's greatest value lies in the eye of advertisers who behold it.

Even before Phelps won his 19th medal, his previous 18 were earning him a nice living: Forbes magazine says his endorsements for Speedo, Visa, Under Armour and others pay him $7 million a year.

So, how much more might his 19th earn him as a pitchman?

Harvard Business School professor Stephen A. Greyser has made a science out of considering such questions.

There are four elements, he says, that determine how much money a medal-winning athlete can "harvest" from endorsements.

-Performance: How extraordinary was the athlete's achievement? "Gold is better than silver, silver better than bronze," he says. "What's the slogan on the Wheaties box? 'The breakfast of champions' Not 'the breakfast of runners-up'." The only thing better than gold, in advertisers' eyes, is a gold medal winner who sets a world record.

-Sport: In what sport did the athlete win? Sports with the greatest viewership have the greatest value to an advertiser, he says. For that reason, medals in track and field, swimming and gymnastics command top dollar. These events also have the attribute, for advertisers, of appealing to both men and women. Medals for the decathlon have a special luster, he says, since the winner of that event has historically been viewed as being 'the world's greatest athlete'.

Medals in low-profile sports typically have lower market value. "Neither archery nor rowing is going to earn you a big payday," he says, though there are exceptions: He cites Kim Rhode, the female shooter who not only won gold in skeet, but set a world record doing it. Moreover, she has competed in five Olympics and has been a medalist in all five. That kind of achievement, says Greyser, counter-balances the relative obscurity of her sport.

-Personality. The medal-winner must have a broadly pleasing personality. That's not to say that he or she needs to be "wimpy or simpy," says Greyser. Depending on the product being advertised, it's sometimes ok for the medalist to be edgy.

-Baggage. The medalist must have, he says, no baggage. He cites the Swiss soccer player who recently was kicked off his team for making derogatory, anti-Korean tweets. "He's unlikely to be generating a lot of endorsement activity," Greyser observes drily.

Returning to Michael Phelps: How much more might his 19th medal earn him?

Greyser demurs giving a dollar amount, but says in general of Phelps: "He's already fairly high up in the pantheon, so this will be incremental for him rather than a plateau shift. He's not going to be going from zero to 100." The swimmer, he says, has already climbed, "to the top of the medal mountain." Now, in addition, he has set a world record for the most medals ever won.

"A wider number of brands may be interested in somebody who has become that distinctive," says Greyser.

In other words, there's likely bigger paydays to come--if Phelps can keep his personal life tidy.