That depends on many factors. A stamp is worth its face value for use as postage. If you have something that is old and is rare, it could be worth some money. Today's First Class postage stamps are 44 cents.
Currently you can purchase it for 44 cents. The Forever stamp is worth the cost to mail one ounce of first class mail. It will always be worth that much regardless of how much it costs at the post office.
In the US the first class stamp for half an ounce was 3 cents. Those stamps are no longer valid for postage, but are worth far more than face value!
A first class stamp costs 46 cents in the US. Each additional ounce is 20 cents.
Up to, but not exceeding, one ounce.
45 cents for the first ounce and 20 cents for each additional ounce (up to a point).
$0.20
.75
45 cents for the first ounce of mail.
US Mail first class postage was .02 per ounce
It depends on the weight of the package or envelope. Letters weighing one ounce or less require a 49 cent stamp called a "forever" stamp.
A Forever stamp is good for one ounce of First Class Postage inside the US.
The cost of a stamp in the US is the same everywhere. It is currently 44 cents for the first ounce of First Class mail.