A horn's value is difficult to assess. It is only as valuable as that one special buyer will pay for it. Its value depends on finding that one buyer and the timing is crucial.
Example . . . years ago I paid probably too much for a gold-plated Selmer Cigar Cutter alto. It was a wonderful player but it looked a little rough. I wanted it - so I paid the price. I doubt if I could sell it for anything close to what I paid for it. My purchase price doesn't mean the horn will draw that price again.
Just review the FOR-SALE section here on SOTW . . . seemingly desirable instruments start out priced pretty high and then the reductions start.
Of course, you can get a ballpark price so as to weigh the cost of an overhaul against the horn's value, or to seek insurance for it, but even then, I wouldn't count on getting that price if you listed it for sale. I suspect an alto of similar vintage would bring more than a tenor of the same vintage, especially early 1930's Selmers. Seems to me that popular vintage tenors are MKVI's and Conn 10M's, whereas popular vintage altos are more diverse. I could be wrong . . . DAVE