“I related most to Batman,” said 56-year-old movie producer Michael Uslan of his childhood comic-book tastes, “because Batman’s greatest superpower was humanity.” Mr. Uslan, like many baby boomers, grew up an avid comic-book reader. But his fandom didn’t stop at 18. As an adult, he parlayed his love of Batman into a successful career in the movie business, producing all five of the most recent Batman movies. His sixth, The Dark Knight, staring Christian Bale and the late Heath Ledger, opens this July.

But Mr. Uslan has a comic-related side career, too, as a comic-art collector. Forget those Cindy Sherman prints, or combing Chelsea walls in search of the next Kara Walker: wealthy ex-nerds looking to make an arty purchase are snubbing the mainstream art world, throwing their bucks at their childhood heros and creating an investment market where there wasn’t one before.

Collecting comic-book art is a pretty new thing. More than one person I spoke with at last weekend’s third annual Comic Con New York, at the Javits Center, told me that people didn’t even really collect comic books until around 1960. (The first comics were published in the ’30s.) The original artwork used to make comic books was routinely treated as disposable, ephemeral. Comic artists and their publishers should kick themselves; today, existing pieces of early comic art are worth hundreds of thousands of dollars, some even over a million.

And what’s still around is selling. Flush with disposable income, ex-nerds who’ve gone on to be the doctors, lawyers, bankers and geniuses we always knew they’d become, have followed their passion and nostalgia for their fictional heroes—and the archvillains that tormented them—to the point where they have transformed comic collecting from a once-mocked American subculture (the mark of an outcast, even in the ’80s, was a love of comics; remember The Lost Boys?) to a highly valuable and well-respected art market. “You’re seeing a shift now,” said author, dealer and consultant Jerry Weist. “A lot of longtime comic-book collectors have moved into art. … That is where the money is.” At a recent auction in Paris an anonymous buyer bought a sought-after piece of art from Tintin in America for a staggering $1.3 million, breaking the record for the most ever spent on comic-book art.

New York City, the inspiration and setting for so many famous comics, is, naturally, also home to many comic-book art collectors. Thirty-six-year-old cardiologist Hari Naidu, a lifelong reader of comics, has been collecting comic-book art since the mid-’90s. Mr. Naidu started buying pieces worth a few hundred bucks and has reached the point where he now buys comic-book artwork that cost tens of thousands of dollars. He is originally from Long Island, but he now lives in a luxury apartment on the Upper East Side.

On one wall of his riverside apartment is a panel of art from a 1980s-era Batman graphic novel that portrayed the early days of Batman’s nemesis, the Joker. When I asked what his most valuable pieces were, he demurred, saying, “Value is tricky to determine, because it’s all based on people’s tastes.” He did say, however, that two of his most cherished pieces are the first appearance of Dr. Doom (archnemesis of the Fantastic Four) and the cover art for Spider-Man #75. Mr. Naidu speculates that these pieces are worth between $50,000 and $75,000.

Joe Mannarino, owner of All-Star Auctions, says that buying comic-book art is the closest thing to a guaranteed profitable investment that’s around these days. “Since this began as a field, the value has never gone down. Every single year … comic art has appreciated in value.” Comic-book art is now so valuable that it may be prohibiting the average comics fan from owning his or her favorite art. “You can’t survive in the hobby these days unless you are making serious money,” adds Mr. Naidu.

AT COMIC CON, mixed among companies with names like Vampire Cowboys Theatre Company and the Gnomon Workshop, were a handful of comic-book art dealers. These companies buy and sell the original artwork for tens and sometimes hundreds of thousands of dollars. Dallas-based Heritage Auction Galleries has sold comic-book art since the early 2000s and had on display a high-grade original copy of the first comic book to feature Superman (very rare) and the original art from the first X-Men, featuring Cyclops, the Beast and Professor X. This latter piece passed through the hands of Metallica guiarist Kirk Hammett and has an estimated value of $40-60,000, according to Heritage Vice President Ed Jaster.

Collectors use four factors—called the four C’s—to determine the value of a piece of comic art: content, context, character and condition. Context refers to the significance of the page on the long-term narrative of that particular comic; content is measured by the dramatic value of the specific page, and can be affected by factors like fight scenes between hero and villain; character value fluctuates depending on the renown of the characters in the piece of art. Condition, surprisingly, is the least important of all the C’s in determining the value of a piece of comic-book art.

Writer and producer David Mandel, who has worked on SNL, Seinfeld and Curb Your Enthusiasm, is a self-identified “nerdy” graduate of Horace Mann and “a voracious reader of comic books.” He lives in L.A. but grew up on the Upper West Side and started collecting comic-book art in the mid-’90s. He described his hobby as “pure nostalgia. … It has accidentally become a great investment.” Pieces that he bought in the mid-’90s—like the original cover art to Giant Size X-Men #1—for $5,000 or $6,000 are now 10 times that amount. But, like so many comic-book art collectors, he claims not to care. “It’s not going anywhere. Put it in my coffin with me,” he said. The monetary value “is for my kids to fight over one day.”

This attachment to the art and unwillingness to trade it around has helped to create a sellers market. “As the popularity grows … the demand grows and grows and grows, and there is just not that much [art] out there,” said Mr. Uslan, who will also have a hard time letting go of anything from his collection. “I don’t think I will ever sell,” he said. “This is a collection that I will pass on to my kids.”

Mr. Weist, the dealer and consultant, mentioned during a phone interview that the original cover art from Iron Man #1 was going to auction this summer, just in time for the movie starring Robert Downey Jr. He expects it to fetch as much as $70,000. “There is just a tremendous dynamic moving this,” he said. “People who bought this stuff in the ’70s got a better return than anything else on the market, with the exception of something like Microsoft.”

At least some nerds are getting revenge.