There are also some good price guides on amazon. But make sure you choose a realistic one. Also, see Related Links, below.
I don't know, but try a site like www.ValueJockey.com. There, you can ask questions about how much stuff is worth.
Gun Broker, Guns America, Guns International
One good site is: http://www.uscurrencyauctions.com/prices.htm At a minimum you have to know the bill's seal color, date, whether there's a letter below or next to the date, and its denomination.
You would have to get it appraised by someone in that field.