Value of old currency
This post is about the value of old currency in the U.S. Believe it or not eBay is probably the best place to determine value of currency you have a question about value on. It is likely, assuming the currency you are attempting to put a number on is not extremely rare, you can find a value most recently paid at auction.
Any currency is only worth what people are willing to pay for it. Auctions are good places to establish base lines in value. Auctions are also a great place to find a recent going price for any old currency you may have. When using search features looking for old currency on the site use keywords such as; "old dollar bill value", or "silver certificate values".
The same rules or guidelines that apply to the value of old coins will be true, in general, in determining the value of old currency or paper notes.
Unlike coins with precious metal content paper currencies have no underlying value in content. At one time paper notes were backed by precious metals but this is no longer the case (no matter what people think and say).
As mentioned above, the best place to look for currency to collect or trade is likely, auctions, dealers, and through other currency collectors. The value of old currency can be very subjective. For example some personal collectors enjoy gathering paper money from certain historical periods while others put value on the paper's art and other more graphic values such as colors and uniqueness.
See the links section and also the side banners in these posts for excellent references for helping you with the value of old currency.