A 1957A $1 silver certificate is not very rare. Silver certificates were U.S. currency redeemable for a silver dollar coin. In 1957, the U.S. Treasury Department only produced silver certificates in $1 denominations, but they printed a very large number of them.
The U.S. Treasury Department printed the 1957, 1957A and 1957B series certificates by the billions, making them all very common and typically worth little more than face value. Serial numbers begin with every alphabet letter and combination, but the only versions that are slightly rare are those beginning with "B" or "BB." However, these notes are still worth less than $50 to collectors, as of April 2015.