Cash--Now, That’s Worth Collecting : Memorabilia: The value of signatures, baseballs and other items depends on a lot of factors, rarity being just one.

If you have the autograph of a movie star, President or sports hero, it may not be just a prized asset for your mantelpiece or coffee table. The signature could also add to your bank account.

“It must be something that people like, like a famous person or [item from a] film,” said Paul Jenkins, director of collectibles at Christie’s. “Then rarity is very important.”

A baseball in mint condition signed by legendary player Lou Gehrig sold for $13,000 at Future Sports & Memorabilia Inc., a New York store.

Signed photographs of Marilyn Monroe, James Dean, Greta Garbo and other cult figures sell for up to $6,500, depending on the image, size and condition.

Letters, books or manuscripts with notations by composers, writers, movie stars and other celebrities may even fetch hundreds of thousands of dollars, if they give a special insight into the person.

But even a plain signature on a piece of paper could sell for $5 to $100.

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“The most valuable are those celebrities who fall into some kind of list, for example, Presidents and baseball Hall of Famers,” said Charles Sachs, owner of the Scriptorium, a dealer of manuscripts and documents in Los Angeles.

“If you win an Academy Award, it makes you valuable beyond your own attraction [as an actor] to a collector” of Oscar winners’ signatures, he added.

This is because collectors will bid up prices to get that hard-to-get signature of President John F. Kennedy, for example, to round out their set of baseballs or letters signed by Presidents.

Kennedy’s autograph can go for $7,000 to $14,000. “He died very young, so it’s hard to get one,” said Howie Schwartz, vice president of Future Sports & Memorabilia. “Truman’s not worth as much because he lived longer.”

An autograph from a reclusive star like Michael Jackson will fetch higher prices than one by a prolific signer, such as Joan Crawford.

Bits of Jackson memorabilia have sold for as much as $18,500, even though some are worth only $250, said Sachs, who put the items up for auction in London and New York.

Photos with signatures of all four Beatles--made rarer due to John Lennon’s death--sell for about $2,500.

But rarity alone is no guarantee of high prices.

“Greta Garbo items are extremely rare, but the value of her letters has dropped by half over the last four to five years” because of dwindling exposure after her death, Sachs said. “When they’re so rare and hard to collect, there are a limited number of buyers.”

On the other hand, Marilyn Monroe’s wide appeal makes her consistently bankable.

“She is the most popular American icon for valid reasons because she intersects culture on the sports, political, entertainment levels by virtue of her marriage, alliances and own activities as a movie star,” Sachs said.

Notoriety also helps but is no guarantee of capital appreciation either. An insurance form signed by O.J. and Nicole Brown Simpson sold for more than $1,000 at an auction by R.M. Smythe in April, helped by media attention on murder trial of the former football star’s wife.

But accusations that Michael Jackson molested a child had little impact on prices of his memorabilia.

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To tap the growing autograph market, companies are contracting sports heroes and musicians to sign limited numbers of products, with the promise that prices will appreciate.

Moreover, these companies provide guarantees that the products are endorsed and signed by the stars they represent. This is important in a market where dealers estimate that 85% of signatures are forgeries.

“Authenticity is key,” said Joel Weinshanker, chief operating officer of Musicom International Inc., which releases limited edition, signed lithographs of pop music album covers.

“People want what they understand. A photo nobody has seen before is less valuable than an album cover, which is universal,” he said.

Musicom’s lithographs of the Rolling Stones’ “Sticky Fingers” and “Voodoo Lounge” album covers, which came out at $2,000 in December, are now worth about $3,500.