What is the value of a 20 bill series 1928 with a green seal?

There are more than half a dozen varieties of this bill, so more information is needed. Please post a new question with the following information:

> What letter, if any, is next to the date. If there is one it'll be A, B, or C.

> The name of the Federal Reserve district that distributed the bill; for ex. Chicago, Kansas City, etc.

Alternately, check the related link below for a price guide.