The top unit of value in the coinage of the late Republic in Rome was the aureus, which prior to Nero was approximately 1/42 of a pound of gold. In 2010 dollars, where a pound of gold is worth about $14,000, the aureus takes on a value of approximately $330.
The denarius, a silver coin, had a value of 1/25 an aureus, putting its value around $13.30. Caesar doubled the legionary’s pay to 225 denarii a year (ca. $3,000). Promotion involved a substantial raise: centurions received 3,750 denarii a year (ca. $50,000).
The sestertius was the basic unit of monetary measure. It was ¼ of a denarius, or about $3.30. It was abbreviated IIS = 2.5 (S being Semi), or ¼ of a denarius (X); this is also written HS. Suetonius reports that Caesar purchased for his mistress Servilia, mother of Brutus, a pearl for six million sesterces, nearly $20 million; the ring Kobe Bryant got for his wife (“the Kobe special”) while he was on trial for sexual assault – when adjusted for gold prices – cost about $11 million.
The as, a bronze coin, was ¼ of a sestertius, or about $0.80. Catullus tells his lover (prob. Clodia, vide infra) that the opinions of all those who condemn their affair are worth an as. In the time of the early Republic – before the unmercantile Romans had any gold or silver coinage at all – the as was a pound of bronze and was the standard unit of measure. Hence in the English translation you will find Livy talking about Romans having such and such a number of asses – a term I initially found first confusing (“Did they measure wealth in donkeys?”), then amusing.
A quadrans was ¼ of an as, about $0.20. We deduce from Horace and Cicero that a quadrans was the typical price of going to the baths in the late Republic. It became emblematic of a small sum. Marcus Caelius Rufus called his ex-wife, the infamous Clodia, a quadrantaria, i.e. a twenty-cent whore.
“Sitting opposite the collection box, he saw how the crowd cast their money into the box; and many rich people put in much. And a poor widow came up, and put in two mites [minuta, dimes, essentially], which make one quadrans [Greek kodrantes]. And calling his disciples he said to them, ‘Amen I say to you, This poor widow has given more than any of the people who gave; they gave out of their excess, but she gave out of her poverty, all she had, all her living.’”