Considerations
The face value of a one dollar Silver Certificate is still its official value as currency. But that dollar today is worth far less than it was in 1928. Still, collectors do put a premium of historic U.S. currency, and Silver Certificates are no exception. 1928 was the year the size of the actual note was reduced from previous issues, which gives it an additional historic value. Ultimately, the value is whatever you can get someone to pay you. Consider, however, that the melt value alone of the silver dollar for which the certificate was originally redeemable is almost $11 with silver at $14 an ounce. The Silver Certificates will usually command a slightly lower price than the actual silver dollar, which means you can reasonably expect to get anywhere from $10 to $25.
Condition
Of course, the condition of the certificate will also affect its value for collectors. Paper currency is graded, like coins, with different levels, from uncirculated down to fine and, lastly, good. PCGS currency uses a popular system that maintains these levels but uses a 70-point system, in which 60 to 70 are reserved for new or uncirculated notes. Having a Silver Certificate professionally graded can be costly, but can boost the value of a superb condition, rare bill. Sellers of such high quality 1928 Silver Certificates can ask for anything from $150 to $300, though it's much more difficult to find buyers at these prices.
Origin
Much of the collectible value of U.S. Silver Certificates comes from the interesting role they played in history. They were first printed in 1878, at which time they were fully redeemable in silver dollar coins equal in number to the denomination of the certificate. The standard silver dollar coin at that time was the Morgan dollar, which has a fineness of .900 and a total silver content of just over 24 grams of gold (0.773 troy ounces). In 1921, the Morgan dollar was replaced by the Peace dollar, of identical composition. A One Dollar Silver Certificate would have been redeemable for one such coin between 1928 and the late 1960s.
End of Redeemability
The announcement that the redeemability of Silver Certificates into silver dollars was officially and permanently ending came on March 25, 1964 from then Secretary of the Treasury C. Douglas Dillon. Actual redeemability ceased on June 24, 1968. As of August 2009, the U.S. Treasury accepts no obligation to issue silver in exchange for the certificates. Interestingly, the certificates are still legal tender and can be negotiated at their face value. The fact that they closely resemble modern Federal Reserve notes meant they probably continued to circulate at face value for some time, though this doesn't occur today.
Features
The similarities between the 1928 Silver Certificate and modern U.S. dollars are obvious. The differences, however, are telling. For example, above and below the portrait of George Washington, the Silver Certificate reads, "This certifies that there is on deposit in the Treasury of The United States of America One Dollar in silver payable to the bearer on demand." No such promise exists on Federal Reserve notes. It is nostalgia for this lost period of sound currency in American history that gives Silver Certificates their allure.