If you are in the United States, and want to send a letter to someone else also within the United States — it doesn't matter which state they or you are in — then the cost of a first-class stamp (to send a standard letter) is 44 cents.

If you have a larger package, so that it is heavier or larger than a standard letter/letter envelope, or if you want extra services such as guaranteed express delivery or certified mail, then postage rates vary. You can find rates at http://www.usps.com/prices/welcome.htm.
it the same price anywhere in the coubtry
How is this a tax question?