Types
Types of coins in the Roman monetary system were the as (made of bronze), the sestertius (also made of bronze), a silver coin called a denarius and, a gold coin called an aureus.
Relative Values
The as was the base unit of Roman currency. A sestertius was worth four asses, and a denarius was worth 16 asses. The aureus was the most valuable by far, worth 25 denarii.
Actual Values
At the time of Emperor Hadrian's rule, a half of a denarius (or two sesterces) would have been the typical cost for a small bag of wheat. At the same time, the average annual salary for a Roman soldier would have been over 200 denarii.
Pricing Units
In early imperial Rome, sesterces were most often used to establish a price. Later on, in the fourth century and after, an item would more likely have been priced in denarii.
Coins as Propaganda
Roman coins generally depicted the emperor ruling when they were minted. Emperors also endeavored to depict war victories and other achievements of their tenures on their coinage.
Fun Fact
After the death of the emperor Caligula, the Roman Senate recalled all coins with his likeness from circulation to be melted down, making coins depicting Caligula a rarity today.