Earlier Roman gold coins (aureus?) apparently were not minted in high quantity. BUT from the reign of Arcadius or Theodosius II in the Eastern Roman Empire (or Byzantine) gold coins were struck in large quantity. Haven't you noticed that not many Roman gold coins are listed for bidding, while that number soars from the reign of Theodosius II and onwards?? In Vcoins if you seach after "Aureus" then only 133 items appear. But for "Solidus" the number is 511 items. (for the record: the number will soar if we include "Histamenon" and "hyperpyron") Gold coins of Thedosius II, Justinian, Heraclius, Constans II, Basil the Macedonian, Constantine VII, Romanus III, Alexious Comnenus, John II Comnenus and Andronicus II are the most common. So, can anybody tell me what happened in the reign of Arcadius/Theodoius II? What is the explanation?
My gut instinct (and others more knowledgeable are welcome to correct me) is that it's at least partly due to the gradual decline in daily small-scale economic transactions as the Western Roman Empire fell. Markets became less frequent, while more people became feudal peasants or otherwise stopped needing small change for daily transactions. Such large-scale transactions as still went on (such as long-distance trade or the sale of luxury goods to t he nobility) required gold coins, so those continued to be produced, while silver and bronze production trailed off.
Massive payments to the "barbarians"? Attila in particular; at its worst the annual payment to him was 2100 pounds(!!) of gold, under Theodosius II.
I agree that it in later roman times it gradually became a one-metal economy. With "haves" and "have-nots". Small change became tiny, worthless, and gradually disappeared. However, with the reforms of Anastasius bronze coins made a comeback and somehow the economy was stabilized.