As of April 2014, it costs $1.15 to send a letter from the United States to Canada. This price applies to letters that weigh up to 1 ounce that are sent by first-class mail.
Additional postage is required for letters that weigh more than 1 ounce or for letters that are mailed using a premium service, such as Priority Mail. A convenient way to determine the exact cost of mailing a letter is the international price calculator found on the USPS web site. If the USPS increases postage rates in the future, the calculator will be updated to reflect the higher postage rates.