Since the value of something is typically the amount buyers are willing to pay, searching on Ebay for that make and model and selecting "completed listings" can give you a rough estimate based upon the sales for the last 30 days. To see the sales for a 12 month period one can subscribe to Terapeak Research which costs $12 per month paid annually or $19 per month with no committment. I kept a membership for several years until the company was taken over by Ebay and you could no longer see the full original ads, but just the description and the price.
^ Completely agree with that. IMO that is how you dependably come up with a "wholesale value" for any vintage horn, though you have to become a bit literate in understanding condition and how it is influencing the results you see.
Knowledgeable shoppers will make their bottom-line pricing based on things they will observe that serve as indicators of other things. Example: if i see a neck tenon slot on an old Conn tenor that
doesn't have bulge, and that looks exceptionally clean in a photo, I can make some other general judgments about other things that are likely to be true of the horn -- same for something like total lack of pulldown. There are quite a lot (maybe thousands?) of things like that in play, though the main things are maybe only a few dozen or so (e.g. pulldown, dented bows, recut engravings, etc. etc. etc.).
I've had SMLs from #9000 and higher, and everything in that range up through the early runs of horns that do not have rolled toneholes merits restoration. They are exceptionally good saxophones. Until the Reference 54 arrived, for me, an SML in the 8000s or 9000s (I can't remember the exact serial number now) had the most dead-accurate intonation of any tenor I'd ever played.
The only thing I think that would lead players to abandon an SML tenor is that the scale's balance is unusual and not like Selmer or Conn -- they tend to be very full and luscious down low, thinner through the middle, and good but not great through the high palms (Conn, Buescher and Martin tenors, for example, are great through the palms, Selmer just OK, and the Selmer scale is almost the opposie of SML in terms of how it's balanced).
They are special, though, and not in the way that some of the lesser known European vintage brands that have developed small followings are special. They are great saxophones.