The usual means of estimating the purchasing power of ancient monetary schemes is to take the price of a relatively stable staple (heh) and examine the prices of other commodities in relation to it. Alternatives like simply converting the equivalent amount of precious metal or finding some denomination of a familiar currency and essentially arbitrarily construct an equivalence are...arbitrary. At the very least they do not accurately reflect the relative purchasing power of various currencies in their social context. The price of bread, or rather the price of grain (partly because it's more basic but also because for large periods of antiquity a sort of porridge was eaten more often than bread by most people, who had to pay to bake the stuff), is usually used as the commodity that is used to fix prices. It is fairly universal, is thought to be relatively consistent, and we have a decent amount of information about the price of grain as compared to other goods.

Unfortunately, the price of grain is still a terrible standard on which to base price estimates. In point of fact, the price of grain is hardly as stable as it might seem, and was in the past frequently nowhere near as stable as it is today. The Green Revolution changed agriculture completely, both in terms of labor and actual yield, and today's grain prices are nothing like the grain prices of the ancient world. Today's grain is produced at a consistent rate and at consistent quality, and the world grain market even in non-industrialized countries is secure enough that natural famines (i.e. famines created by nature, not by human actions like the warlords in Mogadishu hoarding grain) are easily counteracted. In a pre-Green Revolution society the price of grain could fluctuate wildly very rapidly, simply from natural famines. In addition we must consider the existence of man-made famine. In the first century, before Augustus established strict penalties on the manipulation of the grain market, grain traders notoriously created artificial shortages or inflated grain prices in times of plenty. By hoarding grain, restricting the flow of incoming shipments to the public, and trading in futures they could increase their profits and protect against losses, all at the expense of the consumer, who required grain as his main (in many cases sole) source of sustenance. Further, war and storms could break up the flow of grain shipments, creating artificial scarcity and often triggering further manipulation of the market by traders who panicked and prepared for the worst. In 67 under the lex Gabinia Pompey was granted wide and unprecedented military power to engage the pirate fleets operating from Cilicia and funded by Mithridates, which had long been cutting off the grain trade (they even sacked Ostia) and causing grain prices to rise well past unacceptable levels, precipitating a major socio-economic crisis. In this incident we can also see the reverse issue, the sudden "normalization" of the market even before adverse conditions had changed--Cicero claims that the very day that Pompey assumed his command, even though he hadn't even done anything yet, grain prices dropped down to their pre-crisis levels. Some of that must be rhetorical exaggeration, but nevertheless we see often that projected events could screw with the trade of futures and the traders' confidence in their ability to profit, thus changing the market unpredictably, even when the effects of those events (or the events themselves) hadn't come to pass.

Additionally, the value of grain in the ancient world was totally different than today. Grain is and always has been a staple, but it was vastly moreso to more of the world in antiquity than today. Today advances in agriculture, the spread (and breeding) of new crops, and the unprecedented ease with which food can be transported has completely changed the way that even third world countries interact with their food supplies. To the Roman laborer, and his Greek or Egyptian counterpart, grain was pretty much the sole source of food available, and even when other forms of nourishment presented themselves grain, either in the form of bread or some sort of porridge, was always the central part of the ancient diet. Even for the wealthiest individuals grain was not just a staple, it was the staple--lack of grain was equivalent to lack of food, since no other food source was so important. This, combined with the uncertainties of harvest and supply, meant that prices were not necessarily equivalent socially to today's grain. I can go to the supermarket and buy a bag of pre-sliced bread for a couple of bucks that will nourish me and my family for a good week or so at near-starvation levels. For the Roman laborer this was not the case. His grain was provided un-baked, and he either had to eat it that way with water or pay a baker or for the use of a communal oven to have it baked. His grain, being all-important to his diet, was his central daily expense (even discounting rents, which Frier claims were frequently paid daily) and therefore claimed an elevated place not only in his daily life but also in the economy as a whole. It should also be noted that large cities, from which almost all our information on prices derives, were almost always artificially inflated in antiquity. Cities, as a rule, suck their sustenance from their hinterlands, and many if not most truly gigantic ancient cities vastly outstripped the supply capabilities of their hinterlands. Rome's immediate surroundings could feed a population of maybe a couple tens of thousands, and Athens could hardly supply even that many--both cities imported most of their grain by sea (which was cheaper than by land). Alexandria was blessed by the Nile, which allowed her to haul grain from well inland for cheap, and a few cities like Roman Carthage were blessed with extraordinarily fruitful hinterlands. But the origin of grain supply also affected price. So, too, did legal regulations. Since the Gracchi the Romans had imposed certain price restrictions on grain, at first offering 5 modii per month to heads of households at a reduced price, and later for free. This grain was not enough to feed a family, but it reduced the overall cost, and surely had some impact on the cost of grain sold on the market, with which laborers had to supplement their monthly allowances. Unfortunately, we do not have good information on how these changes influenced the market. We know, for example, that in the wake of Clodius' lex de frumentaria there was a famine in the city, which some scholars (Tatum, for example, if I'm remembering what he says correctly) have attributed not to a real grain shortage but to a shortage in capital and a fluctuation in market prices. But we really don't know