They are worth what someone else is willing to pay to state the obvious. the collectible aspect of SBA, Mk6 and 10ms Silversonics etc is a large part of that price. If what is really being asked is whether one is getting value For ones money, well, that is a very different question.
Imo the answer is still quite subjective. One thing about a good collectible sax is they tend to not lose value so as long as one doesnt pay over the odds. If they have the sound you want and the ergonomics work for you getting a horn that is rising in value is a plus.
I have a lovely oldSML GM1 stencil that is a great sounding and playing horn. Isnt likely to go down in value but not being a "name" wont appreciate as much as a known one.
Unfortunately the ergonomics started giving me rsi issues. So i bought. jK shadow which has a the sound I want and great ergos. One day it may be a collectible. Dont know, dont much care as I am not a collector.