U.S. Domestic Postage Rates
date cost per half-ounce March 3, 1863 3 cents October 1, 1883 2 cents date cost per ounce July 1, 1885 2 cents November 3, 1917 3 cents July 1, 1919 2 cents July 6, 1932 3 cents August 1, 1958 4 cents January 7, 1963 5 cents January 7, 1968 6 cents May 16, 1971 8 cents March 2, 1974 10 cents December 31, 1975 13 cents May 28, 1978 15 cents March 22, 1981 18 cents November 1, 1981 20 cents February 17, 1985 22 cents April 3, 1988 25 cents February 3, 1991 29 cents January 1, 1995 32 cents January 10, 1999 33 cents January 7, 2001 34 cents June 30, 2002 37 cents January 8, 2006 39 cents May 14, 2007 41 cents May 12, 2008 42 cents May 11, 2009 44 cents Beginning September 14, 1975, additional ounces were charged a lower rate. Since July 15, 1979, a surcharge has been added for non-standard envelope sizes.
Source: the U.S. Postal Service.
http://www.usps.com/postalhistory/ratesandhistoricalstatistics.htm