The value of a 2 dollar bill varies depending on the condition of the note. Uncirculated notes fetch the highest values followed by notes in EF (extremely fine) conditions, Very Fine notes (those with very few folds), fine condition and very good condition in that order. If the note does not fall in any of these categories, then it is worth the face value only.
1 Additional Answer
The current issue of the $2 bill, printed since 1976, is still in circulation and has the same $2 value as its face. However, if uncirculated, its retail value could be as high as double the face value or $4.