The value of U.S. and G.B. (U.K) stamps can be checked from websites like FindYourStamp'sValue.com and TheSwedishTiger.com. Enter a short description of the stamp in the search bar, and the site will show the current value of stamps that fit that description.
Depending upon the present condition of the stamp, the value varies significantly. A stamp that would be worth $100 in mint condition might only be worth $10 if it is torn. Value sometimes increases with age. The rarest and oldest U.S. stamps are the first stamps of 5 cents and 10 cents issued in 1847. They hold a price tag of $3,000,000 each.