The standard source for collectible stamp values is the Scott postage stamp catalog, which is available in many libraries. A good online resource is the Hobbizine website. As of today, Hobbizine says the value of the Decatur / Macdonough stamp is $0.45 in mint condition, and $0.20 used. Keep in mind that these are values, not prices. They don't necessarily correspond to the price you would pay to buy one from a stamp dealer, or the price you would receive if you were to sell one.