What is the value of a 1928 B US 1 dollar bill?
As of 03/2013 approximate retail values are $5 to $10 for a bill
in average condition.
A nice uncirculated one might sell for around $40
"F" is the highest series letter for 1928 $2 bills, and only for those issued as United States Notes. If your bill has a green seal, you may be looking at the Federal Reserve District letter instead of the series letter which would be by the date. There's more information at the question "What is the value of a 1928 US 5 dollar bill?".