Series of 1882 $20 brown seal notes are making a comeback in popularity. There was a time when most collectors only wanted a five dollar example. However, as the market has advanced and specialized, more and more people are realizing that 1882 twenty dollar bills are scarce. Scarce does not always translate to valuable (but plenty of times it does). The exact value of any national bank note is based on its condition, serial number, and bank of issue. So one note could be worth $200 and a note in the exact same condition from a rare bank could be worth $2,000, or more. You really need to work with an expert when dealing with national currency.
Fortunately, we are experts and we are aggressive buyers. If you have a $20 1882 brown back, then please tell us about it. We would love the chance to look at it and make an offer.