At any given time, gold and silver were usually pegged to each other at a fixed ratio, though the ration changed slowly over time. The ratio can never be constant for too long, due to supply and demand - the two metals are mined at different rates, altering the relative abundances and therefore the supply. Any attempt by a government, either ancient or modern, to keep a bimetallic coinage system in place without taking this into account will eventually devolve into a de facto monometallic currency, as either the gold or silver coinage will be perceived as being undervalued and pulled from circulation - a classic application of Gresham's Law. Different Greek states held different gold:silver ratios, but it tended to fluctuate around 10:1. Augustus fixed the early Imperial ratio at 12.5:1, but it drifted away from this from the time of Nero.
Base-metal coins were a different matter; apart from the very earliest cast bronze coins, nobody seemed to expect or demand that the copper coins contained "full metal value". People were content to let the local government decree that certain coins had certain values - so in that sense, virtually all ancient bronze coinages were token coinages, much like modern coins today are. Again, the Roman system is the best-documented one, with the sestertius being 1/4 of a denarius, or 1/100th of an aureus. The denominations were fixed, but the weights were not; a dupondius did not necessarily weigh half that of a sestertius. The exchange values and names of many of the Greek and Roman Provincial coins are unknown, which is why the catalogues simply label them by diameter (eg. "AE25") instead of giving them a name.
A lot of wisdom in this post. Sap writes from a
Roman Imperial perspective but the same issues, e.g. gradually-shifting AE:AR:AV exchange ratios, use of token bronze,
applied in the
Roman Republic. Whilst the AR:AE ratio is generally taken to have been about 120, in copper-rich
etruria and in the
Roman Republic in the third century BC it might have been as high as 250, and in the first century BC it might have been below 100. In all eras, bronze was often issued lighter, i.e. implying a lower AR:AE ratio for coin than for bullion. Whilst 12.5 is the generally accepted AU:AR ratio at
Rome, one gold issue with clear value marks (XX,
XXXX,LX) that can be compared with value marks on silver (X,V,
IIS) implies an AU:AR ratio of 8.
Of relevance to the original question: there is little documentary evidence for the "
price of bullion" in terms of
money, and what there is, is confusing and difficult to interpret. Indeed there may not have been a liquid market in bullion at times: mines or large-scale bullion supply may have been controlled by the government or publicani (contractors) and transfer prices may have been as much about politics as economics. Of course there
had to be market
supplies of bullion in the cities for flatware,
sculpture and
jewelry, but when mine-supply was often state controlled such prices may have been decreed a little above the intrinsic value of coin so as not to encourage melting of coin,
nor encourage hoarding of bullion! As bronze bullion was inconvenient to
hoard as a store of value, it made bronze coin more attractive for token coin production, as Sap suggests. But, in turn, this at times encouraged the wide-spread manufacture of bronze imitation coin. One can tell a lot from the coinage itself, as well as from the nature of hoarding.
