As of Jan. 26, 2014, the cost to mail a First-Class, 1-ounce letter in the United States is $0.49. One ounce is about 5 to 7 pieces of lightweight writing paper and the envelope.
First-Class postage increases by weight. Additional papers mean additional cost; after the first ounce, it costs an extra $0.21 per ounce, even it's only over weight by one piece of paper. The heaviest a First-Class letter can weigh is 3.5 ounces in a standard envelope and 13 ounces in a large one. Other options for mailing letters include Priority Mail and Priority Mail Express. These options cost more and require special envelopes.