Hi Scott:

My guess is that depends on what keyboard is it, as an example I will use the old organ Hammond B3 with Leslie's either 122 or 147, came out in the early 50's, and now if you want to buy one in mint condition, I have seen people pay triple or more of what originally sold for. The matter of a fact is, Roland, Korg, Nord and even the new Hammond Suzuki still try to make clones of this wonderful instrument, in our T2, we have samples of those, like the Ketron X1, SD1, SD1+, HDs, and other makers, also include samples of setting of this organ.

Another example but for synths, is the Prophet 5 by Sequential Circuits, some model(I do not recalled now) from Moog, and so on.
I will say that there are "Classics" like the electric pianos Rhodes, Wurlitzer, etc....like the guitars too, have certain model that are a HIT, and others not matter how good they may be, do not make the list....and example of that was the Hammond X66, my guess it was to far ahead of it's time.