Courtesy of the “A Day in Pompeii” traveling exhibition, here’s a nice array of imperial Roman currency:
The coinage of Pompeii’s time was based on the bronze as. The lowest value coin was the uncia, worth 1/12 an as, while the highest value coin was the aureus, with a value about 400 times that of an as. Over time this changed, of course, as emperors tinkered with the coinage here and there.
Originally, the value of Roman coinage was primarily based on the intrinsic value of the metal it was made from. But over time, the coinage gradually became more of a fiat currency, as the empire ran low on precious metals, and in response the coinage was increasingly debased. Since the imperial government issued currency made of base metals, while only accepting taxes in gold and silver, the debasement of the currency led directly to bouts of runaway inflation in the later years of the Western Roman Empire.