This is a huge and complicated topic, but since characters in the Dominus series often refer to specific denominations of money, here are the bare-bone basics:
The most valuable Roman coin was the AUREUS. As the Latin name indicates, it was made of gold (purity varied depending on the date of production). Most regular folks never obtained or even saw an aureus. It was minted by government agencies to purchase large stores of grain, to commission war supplies, and to pay taxes or special gifts, among other things.
The second type of Roman coin is known as the DENARIUS, Rome’s silver currency. There were 25 denarii to each aureus. Denarii were often used to pay state wages, especially salaries for soldiers. Remember Gaius bitching about how many denarii he paid for Allerix at the auction? Two thousand denarii for a slave was an outrageous sum, indeed. Gaius had good cause to be pissed off at Lucius.
Denarius minted by Trajan. On the obverse, a portrait of Trajan; on the reverse, an image of the defeated kingdom of Dacia represented as a conquered woman sitting on a pile of armor.
The most common and largest of the Roman coins was the SESTERTIUS. During the second century AD, it was substantial bronze coin. One hundred sestertii = one aureus. These coins were used for many things related to banking and commerce. No matter how large the amount of money under discussion, the Romans usually referred the value in terms of the number of sestertii. So, instead of saying something was worth 2000 aurei, they would instead say 200,000 sestertii. Oh, those whacky Romans. One of the wealthiest Romans to ever live, Marcus Licinius Crassus, was reported to have amassed a fortune of some 200 million sestertii.
Sestertius minted by Trajan. On the obverse, yes… you guessed it – a portrait of Trajan; on the back, Conquered Dacia represented this time as a pacified woman seated with children.
The last coin that concerns us is the AS (plural, asses). The smallest coin produced regularly during the Roman imperial period, the as was made of bronze and was used by common people for everyday transactions, such as purchasing loaves of bread, a shave at the baths, and to pay a prostitute. 400 asses = one aureus. When Gaius ordered Max to toss the street prostitute a silver coin (denarius), he gifted her a generous amount.
As minted by Trajan. Yes, he’s on the obverse of this coin as well. On the reverse, the goddess Fortuna holding her cornucopia (horn of plenty).
So, there’s the basics for Roman coins. You can learn tons more about Roman coins, the Roman economy, prices, and inflation from a variety of popular and academic books as well as online sources (http://en.wikipedia.org/wiki/Roman_currency). It’s a complex and specialized field of study, but it’s also super fun!