As discussed in the article Piano Values, a piano's musical value may well differ from its market value. This is particularly the case when determining antique piano values.
If you're a buyer, antique pianos are where the real bargains are. But there are also a lot of pianos that look good on the outside but are worth less—or even worthless—on the inside.
If you're selling an antique piano, on the other hand, you may well have to settle for a lower price than you may like. Antique pianos aren't like violins, and it takes a connoisseur to appreciate them—they don't appreciate by themselves.
This state of affairs is all the more strange in that pianos are vastly more difficult to maintain than violins. As one piano builder—and former violin maker—once explained to me: a violin has 12 parts. A piano has 12,000 parts. It's therefore orders of magnitude more difficult to keep all those parts in perfect working order, and the more time that goes by the less likely it is that the piano will be in top condition... and that a technician will be able to service or restore your piano to original condition. (How many mechanics would be able to service a first-generation Model T?)
Antique piano values are thus all the more subject to the variables detailed in the article Piano Values. Recall that on average, a piano loses two-thirds of its value (compared to an equivalent new model and adjusted for inflation) after 25 years. However, it's often impossible to compare an antique piano with a new piano by the same manufacturer, since the company may no longer exist. If no apples-to-apples comparison exists, the best that can be done is to compare the antique piano to an equivalent model by another manufacturer. Barring that, it's anyone's best guess.
There is an additional factor in determining value that is relevant only to antique pianos, and that is its action. The so-called English action (actually French, as it was patented by Erard in 1821) involves a double escapement mechanism that allows notes to be repeated very rapidly, without having to release the key all the way. The English action is vastly more complicated than the Viennese action which it replaced. For that reason, antique piano values will be correspondingly less if it involves the simpler action. That said, I have played several antique pianos with Viennese actions that were so perfectly regulated that it was difficult to tell them apart from the faster English action. If you're fortunate enough to find such an instrument (I'm thinking of turn-of-the-century Bösendorfers, for instance) in top condition, you might save considerable money yet suffer little or no performance drawbacks.
Let me caution at this point that most antique pianos I have encountered are in poor condition, and some are outright irreparable. Those that can be serviced need special attention from a specialized piano restorer, lest they be "restroyed" by a well-meaning (and expensive) piano technician.
What all this means for a buyer is that an extremely fine instrument can often be had for relatively little money. My own piano is a 6'10" Schweighofer grand built around 1930 and in excellent condition. I paid only € 5000 for it, restored and delivered, and this when the dollar was at its peak against the euro! Along with Bösendorfer and Ehrbar, Schweighofer pianos were the best of the best among over 150 piano makers in Vienna alone at the turn of the 20th century. (Schweighofer was also the very first Viennese piano, as the company was founded way back in 1792.) Many pianists and piano technicians will argue that they don't make 'em like they used to, as the quality of wood used at the time is better than that available at present, and that the pianos sounded better as a result.
Our discussion on antique piano values also means that many antique pianos are overpriced. Sentimental value is not market value, and it is definitely not musical value. It is always good wisdom to invest in an antique piano appraisal if you are serious about buying or selling an antique piano.