This horn is quite a relic and does not have a lot of value with a missing key.

It requires a very experienced Tech to use spare parts to fabricate a key. Or fall into the

hands on a tech that has other Adolph sax soprano carcass' laying around to use as a donor.

I'd say a good 60 hrs of shop table time. At 35 bucks/hr for the gifted hands of an experienced tech, you are looking at 2K to get it in playing condition. Then you have a primitive instrument that plays out of tune and you can say you have an Adolph Sax soprano. Its the one making the music out of tune.

A Tech with spare parts, that is probably not going to happen. I myself would not pay more than a few hundred bucks in order to take on such a time consuming endeavor. It would be put into a soaking vat of wd40 for months prior to me even attempting to work on dismantling it. It does deserves such treatment after being around since the time of Abe Lincoln.

I look forward to viewing other comments you may receive here. I like to know where the market is. Since I myself live in Cincinnat, I'd love to be able to give you a hands on evaluation. I love these types of challenges, but you are fooling yourself to think it will be a big money maker.

Interesting post. Here at Saxquest, we have restored and sold many Adolphe Sax instruments over the years so I can speak with some authority on the matter. Here are my thoughts:

An orignial Adolphe Sax soprano saxophone circa 1865 is a fairly rare instrument. You see a lot more altos and tenors. As the previous poster pointed out these are definitely collector's instruments and have very little value to a "player". However, the collector's market is strong for early original Adolphe Sax saxophones. The problem that you you are going to face is that collecotr's are also very particular about things being all original. The missing key is not the kiss of death. But even repaired well it will bring down the value of the instrument.  It would be best to take it from an old turn of the century or earlier French Adolphe Sax copy (like Buffet, Evette Shaffer, etc....). There are lots of those out there that can be had for a few hundred dollars, although again, sopranos are more difficult to find. This would at least keep the part from the same period. So long as you can find a key cup of the right size the rest is easy to make. 

All original and fully restored without major past damages, an 1865 Adolphe Sax soprano could bring around $10,000 in the collector's market. However, the restoration cost will probably be closer to $2500-$3000. It can take up to 100 hours to propoerly restore and Adolphe Sax. Soft solder lasts about 100 years so to prevent leaks, the old solder really needs to be cleaned up and the entire instrument reassembled (from the tone holes to the key posts). Also, all of the key posts on these instruments have flanges that go through the body tube. So, under every post is a hole in the body tube that needs to be air tight. 

Besides the missing key, I can also see that your soprano has a fairly significant bend in the body tube. This in and of itself is not too much of a problem but it may be an indicator that more problems are there that can't be seen. You can also see evidense of dents and past damage repairs. Also, it looks like the bell has been cut and resoldered. This was a practice back in the day to bring instruments into the same concert pitch, although it was usually done at the neck end of the horn. There was no standard pitch at the time and Adolphe's can be any where from very low pitch to very high pitch. 

Anyway, as for value of your instrument. Fully restored, it may be somewhere around $4000-$5000 given the past damages and that it will never been 100% original. Considering the restoration costs and past damages, you're looking at an "as is" value in the $2000 or less range. Best of Luck!!

Cheers,
     Mark Overton
     www.saxquest.com 

Here is a tag to  Metzler Music website. He is a brass instrument restorer but is a miracle worker at manufacturing replacement parts for brass, not woodwind, instruments. He may have some insight on collectors that are looking for what you have. The historical instrument collectors are a much different animal than us Players of Vintage American instruments.

http://www.metzlerbrassrepair.com/index.html

Check out his before and after examples. He is a vary friendly man and will be helpful if he can.