As of 2014, the price of a First Class stamp is 49 cents. The price changes frequently based on the needs of the U.S. Postal Service. In 1885, the stamp cost a mere 2 cents.
The U.S. Postal Service introduced the Forever stamp in 2007. This stamp costs the same as a First Class stamp, but are always honored, even when the price of a First Class stamp has increased. For example, a Forever stamp purchased in 2013, when the First Class stamp rate was 46 cents, would be just as valid as the 2014 First Class Stamp that cost 49 cents.