20%, Here is a website that can give the value of your $2.00. It looks like the value is dependant on the issuing bank.
1865 Two Dollar Bill National Currency Value and Information
I also checked Ebay and the prices for this bill is amazing. I know nothing about paper money, but just from the prices here you should be able to command a minimum of $4000. Check Ebay. This should give you an idea of the retail value. Of course if you sell to a wholeseller, it will be less. I still like the 4k number.
Good luck.