It will vary depending on the country and year. Some are more sought after than others. Some were produced in large quantities and so have limited value beyond the face value. You will have to consult a stamp catalog to see if what you have as value.
You can find books on Historic stamps of America 1978 Postal commemorative society on en.allexperts.com/q/Stamps-Philately-1610/2009/2/books-stamps-great-grandfather.htm
You can find 1978 Commemorative stamps of America on about.usps.com/who-we-are/postal-history/stamps-postcards.htm
The ones purchased from the US Postal Service include the stamps.
This company produces a variety of products. Their first-day covers are real enough. I think they make gold or gold-plated replicas of certain stamps, which of course, are not real postage stamps. They offer some stamp and coin combinations that contain real US stamps and coins. I think they all sell at a discount of the secondary market.
The cost of postal stamps are $0.28
In the U.S. Postal Stamps are 42 cents each
It may be purchased online from StampsForSale which has many commemorative stamps. You may also shop at RoyalMail which has a variety of gifts and stamps.
H. D. S. Haverbeck has written: 'The commemorative stamps of the British Commonwealth' -- subject- s -: Commemorative postage stamps
Its a post office, so anything postal from stamps to envelopes.
Email doesn'ts cost anything but when you use postal mail you are buying stamps
How much is postage stamps in 2012
What stamps? First postal stamps were used in 1840.