What you have is called a National Bank Note. They were commonly issued until 1934.
Large size $20 bills retail for $100 and up depending on condition. Higher grade notes sell for much more. From what I can determine, Broadway NB of Nashville, charter number 9774, is probably of average rarity.
National banknotes are a separate subcategory (under US Paper Money) on eBay:
http://coins.listings.ebay.com/Paper-Mon...
If the bill is in average circulated condition, probably about $40. If the bill is in better condition, it may be worth 3 to 5 times face value. If you are interested in selling, let me know. I am a collector of currency and have not seen a bill from this bank.
