Perhaps the first part of this comment might be of interest, although another important aspects of Roman and medieval currencies was also purity, which somewhat implicty resembles the economic and political health of issuing authority, also why some medieval currencies were more priced than others, and generally the varying purity of given coins can be traced to varying economic strenght, like highly esteemed Byzantine coins in early medieval period, significant Lombard recovery in late 7th/early 8th century which coincided with highly increased gold purity, the value of Northern Italian coinage ( Florence, Venice, ... ) in high middle ages, where it was highly esteemed in bordering Lower Austria and Carinthia, and the respective conversion ratios differed based on this quality etc.

While Roman Empire had substantial dominance in terms of coinage, Middle Ages showed significant overlap, so in this sense a wider take is almost necessarily to appreciate and account for significant factors.

Like contemporary economy, ancient and medieval were dependent on the value of currency, and significant economic declines coincided with coinage deevaluation ( weight reduction of either gold or sliver - connected to purity ), inflationary conversion ratios, though by now, I am significantly outside my comfort zone, although I can address some follow-up questions if they will be more up my alley.