Online currency price guide
I often receive requests for the value of coins, and the typical response is to refer them to the PCGS price guide. I also note that it’s the #1 question in the FAQ, which I ask people to read before e-mailing me.
However, when I’ve received requests for the value of currency, I haven’t had much information for them. I’m still on the lookout for a more thorough currency price guide, but for now, US Currency Auctions has a decent list of currency values for US paper money.