And, instrument value (even accounting for the results of auctions and private sales) is a highly subjective matter. Unless you are one who is looking to make a killing by buying low and selling high, your value estimate based upon how a horn plays for you is much more accurate.
I've known naif but well off clarinet players who equate purchase price with instrument quality, who go out and spend big dollars on a top of the line Buffet horn, but who never have bothered to have the horn properly set up to suit their particular physiognomy, resulting in an expensive horn that they fight against constantly. Not fun for them, but it does give them the pride that comes from flaunting the latest R-whatever to come down the Buffet pipeline.
A real world approach would be for the prospective clarinet (or saxophone) purchaser to first learn the basics (on a beginner or (at most) intermediate level horn, then to test out all of the "move up" options thoroughly before pulling the string. And, even then I'd think twice before looking upon a musical instrument purchase as an "investment".
With a horn already in hand, the situation is a bit different. Most of us have blundered onto a quality instrument let go for a song, and are suddenly faced with the dilemma of what to do with it. That's the time to audit the auctions and other instrument sales information, just as you would do with the selling of a used car or a house.
That said, I am not one to "invest" in instruments unless they serve my specific needs. I've got three full Boehm Bb horns (and a Mazzeo, although not the one I would like to have), because I dread the day that my primary Bb horn gets lifted or cracked beyond recovery. When that happens, I want to be able to "step into" another horn that suits me then and there, and not to bother looking for a hard to find replacement that just may not be there.
I've got backups for my primary bass clarinet as well, again with the same issues in mind. But, I have never bothered to own more than one saxophone of a given size, since they are much more easily replaced (and durable - no wood issues there).
And, on those rare occasions when I have "blundered" onto a decent saxophone, I usually pass them on, particularly in the case of tenors (which, God help me, I just cannot abide). Better that someone who appreciates them gets the use of them, than for me to sit on them, waiting on an increase in value.
Now that we have "accurate" on line data on instrument pricing, has anybody bothered to tabulate the effect of the "Japanese collector's market" on the classic Mark VI instruments?