Most fans could only dream of owning a Honus Wagner baseball card, which is the most valuable in the world.

But beginning next month, they’ll be able to buy a share of one of the roughly 50 known Wagner cards that are still in circulation, and which is worth roughly $3 million.

The price of entry: $100.

New York trading platform Rally Rd., which began selling shares of collectible cars two years ago, is now expanding into sports memorabilia, watches and rare books.

Chief Executive Christopher Bruno told The Post his company recently learned that an owner of one of the 50 or so remaining 1910-era Wagner cards was looking for some liquidity, and negotiated a deal.

Rally Rd. will be selling a minority stake in the Wagner for about $500,000.

Beginning in August, subscribers to the Rally Rd. app can start buying as many as 4,000 to 5,000 shares in the card, with each share representing about .00004 percent of the card, Bruno said.

There is no other competitor that sells shares in rare collectibles other than art, he added.

A seller might prefer to go through Rally Rd. rather than dealing with an art auctioneer like Sotheby’s because Rally Rd. charges much lower fees, which can be around 2.5 percent on top of the selling price, Bruno said.

Rally Rd. also has plans to sell shares in the signed contract between Muhammad Ali and Joe Frazier from their 1971 Madison Square Garden fight, which might be worth between $50,000 and $100,000.

There are plans to sell shares in a first edition of “Harry Potter and the Philosopher’s Stone,” one of about 200 of the books in existence.

“We have largely a millennial audience,” Bruno said, noting that the app’s users are age 27 on average.