The value of a 1934 series $20 bill varies depending on the condition of the bill and the bill's serial number. Generally, any 1934 series $20 bill not in pristine condition is only worth the face value of $20.
1934 series $20 bills are extremely common. Bills in perfect condition typically command a slight premium. Notes with low serial numbers, such as those with six or seven leading zeros, are definitely worth more, while bills with five leading zeros are possibly more valuable. Those $20 notes that have a star symbol at the end of the serial number may be worth more than $100.