It is an antique, like most of us on this site. The value of the instrument is subjective, meaning it depends on why one would want the instrument. For me, it wouldn’t be very valuable since I have 2, and their only purpose is to be a time witness in my mini-museum of survey instruments. To someone that cut their teeth turning angles on that instrument would value it higher than a 20 year old that has only seen them in a book.
For someone not in the surveying, mapping, or navigation world, it would be an interesting paper weight.
If you are looking to get rid of it, you can try to sell it on ebay & clear $40 or you might try contacting a local surveyor that may put a higher value on it. Another option is to donate it to the local college or university, provided they have a civil/survey class or two, and write the donation off of your taxes.
Nice find. I hope you enjoy looking up the history of the unit.