The auctioning of a tiny sled named Rosebud for $60,500 suggests that prop art could be the newest form of gold in the hills of Hollywood. ''Rosebud,'' was the word the dying Citizen Kane gasped out, referring to a cheap sled taken from him when he was a child.

The final shot in the great 1941 film shows what looks like the real Rosebud being tossed in a furnace by indifferent workmen. In fact, says Sotheby Parke Bernet, three balsa sleds were used for the film and what it was putting up for sale is the last survivor.

That is why producer Steven Spielberg eagerly bid $55,000 plus a 10 percent buyer's fee. But is the sled the only authentic Rosebud? Here comes the interesting catch in prop art. A Port Jefferson film buff claims that he won the real Rosebud as a contest prize back in 1941.

Wherever there is money in collectibles, there are bound to be fakes and arguments over authenticity. Who will claim to have found the trenchcoat worn by Humphrey Bogart in ''Casablanca''? And how will he be able to prove it? There are also bound to be those who bet on the future of the market. For Mr. Spielberg himself now to autograph the harpoons really used in his own modern classic, ''Jaws,'' would be prudent, though some might say improper.