To many factors to define here: How old is it? What condition is it in? Is it used or mint? Exactly what stamp is it? Consult a stamp catalog at your local library. The introduction will explain...
In my opinion, 'Old stamps' are stamps that have been issued from about 1850 to 1935. 'Old stamps' range in value from a few pennies to millions of dollars depending on which one you find or have....
The worth of a stamp depends on many factors which could cause a stamp to be worth pennies to hundreds, thousands into a million dollars. Most issues were printed in the 100's of millions of stamps...
Many countries have an "ugly" policy, to made some "replica" in gold of stamp. Prices depend on market, if someone is selling or buying. From my experience, that is more that 30 yr in philately - I...