1) The value of the denarius was clearly subject to inflation, and so varied greatly. You could say "how much was the denarius worth of October 1st, 10 AD?", but that value would have changed by the next day. In fact, around the time this edict was proclaimed, clearly the value of the denarius was inflating rapidly.
2) The only way you could come up with a relative value for even one day is by looking at how much comparable goods cost. The thing is, are there any comparable goods? The average "consumption basket" (used to calculate the inflation index in modern times) was drastically different back then. The standard of living of people around that time was incomparable.
A kilogram of corn, which you might think would be a good item to evaluate relative prices, was harder to come by back then. Relative and absolute scarcity of resources was different.
That is a complicated issue. If it's not meaningless to ask what a dollar 50 years ago was worth in present terms, then it isn't meaningless to ask about further back. E.g. the answer would appear not to be a trillion, or a trillionth.
However, while this seems to be the view of most economists, I think it's meaningless to ask even what the value of a dollar 50 years ago was, because we are so much richer now. A dollar now can buy you computing power that would have been unthinkable then, cures for diseases that were then incurable, and so on. Economists deliberately ignore immensely valuable new innovations till their prices stabilize (= decline dramatically), since otherwise the CPI would jump around wildly. But this means the CPI greatly understates the growth in wealth.
I found a National Geographic Moon Landing Special from 1969 the other day. It being more than a decade older than me, I was amused at the adverts in the front.
Watches with "electric" (rather than 'electronic') precision, that run for a year on a power cell and don't need winding (gasp!).
A stereo where the 'stereo' ability was the main feature (woo!).
Polaroid cameras advertising all the brilliant things you could do for your business by having a near-instant photo (You mean they didn't bother inventing a digital camera to take to the moon? What were they doing?).
A cadillac, an oldsmobile, a steel framed suitcase, a Hassleblad camera crowing about auto-wind-on film, an insurance company covering an elephant on a raft, a dishwasher...
A mere 40 years ago, and a lot of the products have an air of being low-spec, basic, simplistic, even. Growth in wealth? Aye.
We define money in terms of wealth & then are forced to define wealth in terms of money. Even using item baskets as a unit of wealth is really doomed to imperfection. All you are doing is attempting to standardise wealth. That's what money try's to do.
In any case, while I disagree that it is meaningless to make comparisons, I think that it is meaningful to discuss the value of money. Wealth is not a precise unit. I'm not at all sure that makes it meaningless. But I'd rather avoid defining it as that which a basket measures.
What remains comparable is how much humans need to eat in a day. So it could be meaningful to work out how many people a day's wage could feed back then and use that as a baseline to calculate the imaginary USD/DRI exchange rate.
Think of it as "scarcity distribution" if you will. The typical food basket cost a different amount then than now. And there are plenty of times when most people could barely afford to eat (e.g. the early industrial age, when most factory workers were barely paid a living wage).
A good example of the differences that make this discrepancy so large is the price of salt. Just now, the department in charge of road gritting in the UK has announced that it's out of gritting salt, so they're planning to use table salt instead. Flash back two thousand years ago, and salt was so precious that it was used as a currency to pay soldiers (hence the name "salary").
Other commodities suffer from similar differences, not only with 2000 years of distance, but even closer to us. As pg pointed out, many things which are available today for a dollar were unthinkable even 50 years ago. All these discrepancies make it impossible to compute any sort of exchange rate between dollars and denarii. You can make one up in your head, but it's pure fantasy.
I don't deny the difficulties at all and these difficulties are hotly debated among economists even today when comparing say China or Cuba with the US. There is no completely satisfactory solution, but concepts like purchasing power parity are not completely meaningless even when different types of goods are consumed.
I agree that it gets very vague when societies are as different as ancient Rome and a modern industrial country. But knowing what share of a person's income is spent on biological necessities (basic food and shelter) is meaningful and can be expressed in dollar terms to make it more understandable.
It is hard to make a good comparison because the economy was so fundamentally different. Our modern kg of corn could be anywhere from pennies (for animal feed) to several bucks at Whole Foods. Which do you compare against?
(In fact, what we call corn is a new world crop, unknown in Europe until the 16th century. The word would have referred to barley or wheat before that.) So not even the most basic foodstuffs were comparable.
Wages are a much better basis for comparison. Unskilled, medium-skilled, and high-skilled jobs are pretty comparable now and then.
Using the prices listed and assuming a 10 hour work day, one denarius was worth:
1/4 of a decent beer, or 1/2 of a Egyptian beer
1/2 an ounce of Pork Mincemeat
1/12th of an Italian pound of Venison
1/2 a bath
With a 10 hour day, laborers made:
2.5 denarii an hour for farm labor
5 denarii an hour for carpentry
2.5 denarii an hour for data entry (scribing)
1.5 denarii an hour for entry level teaching
(assuming 10 pupils and 10 hour workday
