This article is from the alt.usage.english FAQ, by Mark Israel email@example.com with numerous contributions by others.
This expression meaning "to contribute one's opinion" dates from
the late nineteenth century. Bo Bradham suggested that it came from
"the days of $.02 postage. To 'put one's two cents' worth in'
referred to the cost of a letter to the editor, the president, or
whomever was deserving". According to the Encyclopaedia
Britannica, the first-class postal rate was 2 cents an ounce between
1883 and 1932 (with the exception of a brief period during World War
I). This OED citation confirms that two-cent stamps were once
common: "1902 ELIZ. L. BANKS Newspaper Girl xiv, Dinah got a letter
through the American mail. She had fivepence to pay on it, because
only a common two-cent stamp had been stuck on it." On the other
hand, "two-cent" was an American expression for "of little value"
(similar to British "twopenny-halfpenny"), so the phrase may simply
have indicated the writer's modesty about the value of his