The collector value of any U.S. banknote depends on its condition, type, variety, series date, denomination and production totals, so no single overall answer is possible. The United States government began issuing its own paper money in 1862, and issued many different designs for each denomination over the next 66 years. In 1928, U.S. paper money was reduced in size by 25 percent and the designs of each denomination were standardized as to the portraits on the face and the emblems or monuments on the back. You won't find large-sized pre-1928 U.S. currency in circulation, but any you run across will be worth more than face value and should be evaluated by a knowledgeable person.
This is the single biggest factor affecting the value of a U.S. banknote, says note dealer Heritage Auction Galleries. Uncirculated notes look as new as the day they left the printing press, crisp and clean with no stains, soiling, creases, folds or tears. As a collectible, these command the highest premium above face. Notes with creases and folds, but little soiling, may grade as Extremely Fine or Very Fine and can command a significant premium. Notes showing substantial soiling, creasing, folds and other signs of circulation wear but no major tears, stains or other damage may grade Fine and command a modest premium above face. Notes that are limp, dirty, frayed, torn, taped together, heavily worn, stained and damaged generally are worth only face value unless from a scarce issue.
U.S. Banknote Types
Small-sized U.S. currency comes in five major types, distinguishable by the color of the Treasury seal to the right of the portrait. The three most common types bear green, red and blue seals. Green seals are Federal Reserve Notes, and are the only type still being printed. Most green seals, including green seal $2 bills, are worth only face value. Some green seal notes from Series 1963 and earlier may command a significant premium if in uncirculated grade or from a scarce series. Red seal notes are United States Notes, last issued in 1971. The $2 and $5 red seals are considered common, and are worth only a few cents above face value unless in uncirculated grade or from a scarce series. All $1 red seals and $100 red seals are scarce and carry a significant premium above face value. Blue seal notes, last issued in 1962, are Silver Certificates. Issued in $1, $5 and $10 denominations, most blue seals in average circulated condition are worth only a few pennies above face value. Crisp uncirculated specimens and certain scarce issues can command significant premiums.
Two Scarce Types
Notes with brown and orange-yellow Treasury seals all are worth significantly more than face value, says Heritage Galleries, so you're not likely to encounter them in circulation. Brown seal notes are National Currency notes, issued 1929 to 1935, and are distinguished by a bank name to the left of the portrait. All are worth a significant premium above face value. Notes with an orange-yellow seal to the left of the portrait are Gold Certificates, issued 1928 to 1933. All command a substantial premium above face value. Gold and Silver Certificates can't be redeemed for gold or silver coins anymore, but are still legal tender at their face value.
U.S. Currency Varieties
The most common variety in U.S. currency is the star note. These notes have a star printed at the end of the serial number instead of the normal letter. Star notes are printed in a separate press run and used to replace spoiled notes from the regular press run. All star notes regardless of seal color have some premium above face value, ranging from a few pennies up to hundreds of dollars, said Heritage Galleries. Another variety is the special Series 1935A silver certificates issued during World War II in Hawaii, the Pacific Theater and North Africa. These notes were easily distinguished from regular U.S. currency so they could be declared worthless if large amounts were ever captured by the enemy. The Hawaii notes bear a brown seal and the word Hawaii overprinted on front and back. The North Africa notes had a bright yellow Treasury seal instead of the normal blue. These have significant premium value in better grades.
U.S. currency is dated by series year, not year of printing, according to the U.S. Treasury Department. Unlike with coins, the year date on U.S. currency doesn't change each year. Two events will change the series year date--a major currency redesign or appointment of a new Secretary of the Treasury. The series year gets a suffix letter when a new U.S. Treasurer is appointed. A new Treasury Secretary or Treasurer requires a change to the official signatures printed on each note.
U.S. currency since World War II has been issued in denominations of $1, $2, $5, $10, $20, $50 and $100, and these denominations are the most widely collected. Until 1946, the U.S. also issued green-seal Federal Reserve Notes in denominations of $500, $1,000, $5,000 and $10,000. All the denominations above $100 have some premium above face value, says Heritage Galleries, so you're not likely to encounter these in circulation.
As with any collectible item, certain issues of U.S. currency had much smaller press runs then others, creating rarities such as the Series 1933 $10 Silver Certificate. Any serious currency collector needs a reference book to identify the low-production rarities. "The Standard Guide to Small Sized U.S. Paper Money, 1928 to Date," by Dean Oakes and John Schwartz, is the standard reference book in this field, says Heritage Galleries. It's available from publisher Krause Publications and many other booksellers. It may also be available in your public library. Even if a note has no collector premium, though, all paper money issued by the U.S. government is still legal tender you can spend at its face value.