An old T-shirt once worn by the singer is expected to fetch $20,000

You want a piece of Britney Spears? It won’t come cheap.

Seven articles of clothing once worn by the pop star are among the pieces being offered to the public by online auctioneers Gotta Have It! as part of their upcoming Rock & Roll Pop Art Auction.

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One of the featured pieces is a T-shirt worn by Spears, 26, while she was a cast member on the New Mickey Mouse Club and signed by her costars, including Justin Timberlake, Christina Aguilera, and Keri Russell. (Spears had given the shirt to her grandmother.)

“Something like that has never come up for auction before,” Gotta Have It! owner Ed Kosinski tells PEOPLE. “We expect it to go for between $15,000 and $20,000.”

Other Spears items include costumes from her “Drive Me Crazy” and “Oops, I Did it Again” tours, a blue and white sexy Santa dress she wore in a Christmas ad for Pepsi in 2000, and the orange coat and pants she wore to the Billboard Music Awards that same year.

Also on the block: a pair of jeans that Marilyn Monroe wore in the film River of No Return, which Spears had received as a gift from Tommy Hilfiger. Kosinski expects them to sell in the range of $60,000 to $80,000.

Proceeds from the sale of the Spears memorabilia, which was provided by Tri Star Sports and Entertainment Group on behalf of the singer, will go to a charity of Tri Star s choosing. The company s owner, Lou Taylor, is a spokesperson for the Spears family.

In addition to the Spears items, more than 500 lots will be offered – everything from an Elvis jumpsuit to Madonna’s 1987-1988 datebook.

The datebook is “all hand-written and what she did day-to-day – if she had dinner, if she had rehearsals,” says Kosinski. It even includes phone numbers for Madonna’s then-husband, Sean Penn, and her Dick Tracy costar, Dustin Hoffman. Estimated selling price: $6,000 to $8,000.

The online auction will be open from July 21 until Aug. 6.