Brady Hill used his skills for buying and selling baseball cards to purchase his first car and to pay his way through Louisiana State University.

Now that he’s the president of Greensource, one of the country’s largest T-shirt printing companies, Hill is again devoting time and financial resources to his hobby.

“Seven or eight years ago, I went to see these cards I had in my safe deposit box that I was going to keep forever,” said Hill, 47. “I said, ‘This is cool.’ Then I looked on eBay and said, ‘Wow, this is legit.’”

Prices have certainly risen for collectible trading cards. Over the past decade, as the S&P 500 has roared back from the 2008 crash, an index of the top 500 baseball cards has done even better — beating it by more than double.

Hill and his wife have put about 20 percent of their net worth in baseball cards, he said. They did so for both investment and aesthetic reasons.

“What’s more fun?” Hill asked. “Having a Babe Ruth rookie card or shares of stock where one false move and it goes down?”

The cards make for remarkably portable assets. If Hill held his 1916 Babe Ruth card in one hand and his 1952 Mickey Mantle in the other, he would have about $1.2 million pinched between his fingers.

Those sky-high values mean that collecting cards is no longer child’s play; it’s a big-money realm of affluent collectors who have their eye on returns. And Hill is part of a growing group of collectors who see their cards as investments, even if the type of investment they are — artwork or security — is debatable.

PWCC, an auction site for trading cards of all kinds, is set to introduce a series of indexes modeled on the S&P 500 to help collectors track the value of their cards and understand what their portfolio is worth.

The company has three indexes of the top-trading high-value cards: the top 100, 500 and 2,500. The indexes are based on auction sales data from the past 10 years.

The top three are the 1952 Mantle, like Hill’s; a 1954 Hank Aaron card; and a 1933 Babe Ruth card. Over the past decade, the Mantle card has appreciated 590 percent, the Aaron card 829 percent, and the Ruth card 305 percent.

PWCC, which is one of 10 or so big card auction sites, helped collectors sell $50 million in cards last year.

Like any investment, what goes up can go down, and investing in trading cards has its own risks. Huigens pointed out that many cards in the top 500 have fallen in value. There have also been several card bubbles over the past three decades.

He said a small group of collectors with about $20 million among them ran up prices in 2016, but the broader collecting market did not follow. “They put all their money into one card and not others,” he said. “They created outliers they couldn’t maintain. The market has had to reassess itself.”

The other driving force in turning a card hobby into an investment opportunity has been the acceptance of a single recognized grading system for the condition of the cards. An independent company called Professional Sports Authenticator will rate any card and give a grade from 0 to 10, which assesses a card’s condition and, in most cases, determines its price.

The rising interest in cards as investments, though, is not spread evenly through the collecting universe; it is strongest at the highest end. Huigens said the rate of appreciation — and high prices — slows beyond the top 100 cards.

But like any investment, you need a strategy. Hill said he focused on cards where “supply meets demand.”

“You can’t just buy rarity,” he said. “If it’s the only one, everyone is going to know what you paid for it. There is no chance for appreciation. You have to wait for someone to come to your door and wrestle it away from you.”

The PWCC indexes shy away from the rarest cards because, Huigens said, they do not trade often enough, and the indexes are a measure of cards that are liquid. One parameter he set was all the included cards must have been sold at least 10 times in the last 10 years, with at least two sales in the last two months.

PWCC also uses the grade — not just on the player, the year and the maker — when picking cards for its indexes. The top card in the index is the 1952 Topps Mickey Mantle in Grade 8 condition, which is worth about $400,000. There are only 33 known examples of this card in the world, Huigens said.

(There are more expensive examples of the 1952 Topps Mickey Mantle; a Grade 10 card is worth around $10 million — but there are only three of those in the world and they don’t change hands often, so they are excluded from the index.)

Sitting atop the excluded list is a 1909 T206 Honus Wagner, once owned by the hockey Hall of Famer Wayne Gretzky. (Gretzky’s own 1979 playing card is ranked No. 44, at a price of $25,000, the second-highest ranked hockey card behind Gordie Howe’s.)

To collectors, this Honus Wagner card has a mystical aura. It has been in the collection of Ken Kendrick, managing general partner of the Arizona Diamondbacks baseball team, since 2007. He reportedly bought it privately for $2.8 million, but its value now would be far higher — should he wish to sell it.

Kendrick owns the top 25 cards in the finest specimens available. Yet he remains a collector’s collector. In an interview, he said that in the 1990s, when he stepped back from running Datatel, the technology company he founded, his first priority was to complete the sets that he and his brother created when they were boys.

He needed 40 cards for the full 1952 set, he recalled, and he went to work.

“During that time, I began to learn about the high-value cards with great grades,” said Kendrick, whose Diamondbacks will host the Colorado Rockies on opening day on Thursday.

He made it his focus to acquire the top 25 cards in the highest grades.

“The cards that I own are more akin to works of art,” Kendrick said. “They’re valued like rare paintings because they are so rare, and they do carry very high price tags if they’re resold.”

Huigens said Kendrick’s Honus Wagner and Mickey Mantle cards were together worth more than $20 million.

Would-be collectors need not lay down so much money to get started; the prices for high-quality cards can be as low as the hundreds or thousands of dollars. “I have cards worth $600,000 or $700,000,” Hill said. “But I also have another 1,000 cards that don’t add up to that price.”

He said he constantly bought and sold cards, both to realize gains and to fill holes in his collection.

Baseball dominates card collecting, but other sports have climbed high in the PWCC index ranking, usually with cards of the sports’ best players. The highest ranked non-baseball card comes in at No.15: a Grade 10 rookie card of Larry Bird and Magic Johnson from 1980. It is worth about $75,000.

The highest ranked football card is a Joe Namath Topps card from 1965. It’s No. 37 and worth $35,000 in a near-perfect grade of 8.

Younger buyers are driving card collecting in a different direction. A first-edition set of the 1999 North American set of Pokémon cards with the top grade recently sold at auction for $98,000.

“The market has changed in terms of how the youth is involved or exposed to it,” said Joseph Orlando, chief executive of Collectors Universe, the parent company of Professional Sports Authenticator. “For a whole generation of millennials, Pokémon was a game with universal, global appeal.”

For most collectors, a love of the hobby remains at the center. Kendrick said he was working on improving the quality of the cards in his 1952 Topps set.

“I have a full set of cards from that year and extra cards, but they’re not the most highly graded,” he said. “I’ve moved up the rankings, and I’m No.2.”

As for No. 1? “A lawyer in Alabama,” Kendrick said. “He knows I’m gaining on him.”

© 2018 The New York Times Company