How Much Does A Postage Stamp Cost For 2017. Effective January 22, 2017 the cost of a first class (1) ounce letter sent through the United States Post Office increased two cents from $0.47 to $0.49 for letters mailed within the United States. Reversing the $0.2 price drop from $0.49 to $0.47 effective April 10, 2016.