Cellos and violins-even those not played or owned by virtuosos-really do gain value in ways that most guitars don't and guitars do gain value.
Part of the reason violins, celli. etc gain in value is because besides being tools for music-making, they are also considered 'art objects.' The Old Masters (Stradivari, Amati, the Guarneri family) were just dudes who were trying to pay the bills and feed their families. Our reverence for their pieces have elevated them to the status of a Rembrandt or Michelangelo. The rarity of these instruments adds even more value, as only a fraction of their total output remains. We in the biz call it "the dead guy factor."
As each Strad sold rises in price, it affects the market all the way down. During the 80's & 90's, there was an explosion in the Chrsitie's and Southeby's market for rare old instruments, and the whole thing took off. My former teacher bought a Matteo Gofriller in 1976 for 20K. Today, it's valued at around 300 grand.
A modern Italian like mine (Naples, 1905) was still affordable 20 years ago, but I'd never be able to afford it if I was in the market today. 5 years after I took possession of it, I had it reappraised for insurance purposes. It had already appreciated in value by half the purchase price.
Rare bows are even more insane. 19th c. French bows are the Strads here. A stick carved by someone like Domenique Peccate was affordable in 1990- maybe 2500-3K. Christie's has one up for a pending auction, and it's estimated to go for between 30 and 50K. And the Granddaddy of bow makers, Fracois Tourte... well, Christie's just sold on of his sticks for... are you sitting down?... ...
$182,500. Antiquities markets are wild. It's now to the point that rank & file orch musicians can no longer afford to get in the game... and hot young aspiring soloists with promising careers are playing on instruments loaned to them by ultra-rich benefactors, who own the instruments as investments. The story behind my purchase is one for its own thread, but let's just say that it would never have happened if a number of pieces hadn't fallen into place just so.