The primary factors that affect the value of a wheat penny are the rarity and condition of the penny. Some wheat pennies have unusual qualities that increase their value.
Wheat pennies were manufactured from 1909 to 1956, and generally, the older coins are worth more. Circulated coins, which are the ones that were used to buy goods and thus show more wear, are worth less than an equivalent uncirculated coin. Some years had large batches of unremarkable coins, and these pennies are usually worth less than other coins. However, all wheat pennies are worth more than their face value.
Some wheat pennies are uniquely valuable. This is often caused by a mistake in the manufacturing process. For example, 1922 wheat pennies without a mint mark are valuable because it was caused by an error. Others are valuable due to their unique place in history. Many wheat pennies from 1943 and 1944 are valuable because they are made from bronze or steel instead of copper, which was being used to make weapons and equipment for soldiers fighting in World War II.
The market itself also determines the value of a wheat penny. When the coin market is strong, interest is high, and people spend more money which makes the coin more valuable.