While many factors contribute to a coin's value, even low quality, circulated wheat pennies are worth approximately 4 cents more than an equivalent modern penny. The primary reasons have to do with changes in the minting process over time, scarcity and collector demand.
The coin known as the Lincoln Wheat penny was minted from 1909 until 1958, going through a number of minor detail changes throughout its circulated life. Some years, the unique characteristics of some mintings were exceptionally valuable, such as copper pennies from 1943 and steel pennies from 1944. These examples are unique for their date, due to the change from copper to zinc during World War II, and the eventual change back to copper from zinc when U.S. involvement in the war ended. Modern pennies began printing in 1959, with the Lincoln Memorial displayed on the reverse side, in place of the older wheat chaff design.
Certain key dates and characteristics can make a modern penny worth a considerable amount, though even the most highly sought after modern penny is still less valuable than the most desirable Wheat pennies. Even among modern pennies, there have been significant changes. A good example would be pennies minted prior to 1982, when the U.S. Mint began using zinc planchets in place of 95 percent copper plates.