If you are describing a small "S" next to the bow in Washington's hair, that's the mint mark indicating your coin was struck* in San Francisco.
The bad news is that San Francisco only makes proof coins, specially-struck coins that are sold to collectors at a price well above face value. But because your coin has been taken out of its packaging and spent, it loses most or all of that extra value.
If you're describing a different kind of letter, your coin has been altered by someone with some metalworking tools, and is only worth a quarter.
(*) FWIW coins are said to be struck rather than "stamped".
Face Value
Only 25 cents, spend it
Only 3 are know to exist. Its worth about 10 million dollars
a quarter
what is a 1989 10,000 pesos worth
Just to make sure take it to a dealer or collector for and opinion, but it's likely from a filled die that caused the mint mark not to strike up.
25 cents.
stamped in the right side of the tunnel
Mis-stamped coins often have value over and above theri face value for collectors. However it is difficult to say for sure about the value of a misprint coin without actually looking at it. Condition of the coin is very important in such cases, if it is mint condition it may be quite valuable, however it if is worn it may not be worth all that much.
All the Washington Quarters minted in the year 1989 carried a mint mark: "P," "S," or "D." The P and D minted coins are worth in MS60 condition: 75 cents; the S coin was minted only in proof and its value is: $3.00.
Kennedy is on the HALF dollar. A 1994 half is worth 50 cents.
1989