According to soundgenerator.com, Mark Twain's guitar is worth a lot:
A guitar belonged to one of the most famous literary figures in American history has been valued at over $15m. The 1835 Martin acoustic...was acquired by renowned guitar collector Hank Risan in the mid-nineties, replete with its original coffin case and an authentic shipping label dated 1866:
"It's one of the best-sounding guitars I've ever played," says Risan. "It still retains its original finish and everything is original to the guitar, except the bridge, which was made in 1850 or 1860. The guitar has a great provenance," Risan worked alongside the Mark Twain Project at Berkeley University Collage to authenticate the guitar, bringing in a forensic team from the Bureau of Engraving. The collector also established the online exhibition "The Private Life of Mark Twain" at the Museum of Modern Instruments (MoMI), which also showcases an unpublished Twain poem titled "Genius."
Twain purchased the rosewood Martin guitar in 1861, the dawn of the Civil War, for ten dollars. [That's the only good investment he ever made. -JH]
That's pretty good, but Edgar Allen Poe's pennywhistle goes for $18 million, and he only paid two cents for it. Also, I wonder how much Hal Holbrook's guitar would be worth. And what about George Segal's banjo?