I saw this question posed in an online forum once and it has come back to my mind every now and then. It’s a silly question in the sense that we don’t have time machines. But I can also see how someone writing a time travel story might find it interesting.
On the one hand, the main factor that determines a coin’s value is its fiduciary value -- that is, how much exchange value it is acknowledged to possess. The same goes for ancient coins. On one level it is the component metals that give it value. But the stamp of a Ptolemaic king or a Roman aristocrat on a coin is what guarantees that value and allows it to be used in legitimate transactions.
Modern stamping would offer no fiduciary value at all in an ancient market. So the main factor in their value would be the bullion value: the value of the metals composing the coin.
Pretty much all modern coins are alloys. And they’re alloys that were not standardly used in antiquity. In many modern currencies, low-denomination coins are even more base, made of copper- or nickel-plated steel.
This means that the practical value would be the value of the component metals, minus the cost of extracting those component metals. This would very likely result in a net negative value. You would literally need to pay people to take the coins from you.
If you could magic the metals out of the coin and convert them to bullion, then you’d get some positive value. How much value?
In modern coins, the only value worth looking at is the value of the copper (and modern coins tend to contain about 4 to 8 g copper). This was the basest metal used in ancient coins; but even in antiquity, the value of copper/brass/bronze coins was primarily fiduciary, not intrinsic. Good coinage that could be used in international trade was silver. Gold was for ultra-high-value exchanges and storage. For domestic use, copper and bronze coins were dominant, as they still are today.
The upshot is: in an ancient market you might be able to fob off modern coins to someone as a souvenir, but then the coins’ value is going to be a function of your haggling skills rather than any intrinsic value.
Below I give some sample coins. To work out the intrinsic values, or ‘bullion’ value, is easy if we wanted to sell the materials in the modern era: we simply look at the current price of copper. For that column, I’ve gone for a price per kg of EUR€5.41, USD$6.06, GBP£4.81, and NZD$9.43.
How to reckon the ancient price of copper, though? Well, in the early Principate, during the reigns of Augustus and Tiberius, the material that the sestertius was made of, brass or orichalcum, was reckoned as double the value of copper. Sestertii of that time range between about 22.5 to 26.5 grams. So, for want of anything better, I’m going to set the exchange rate at 1 sestertius = 24 grams of orichalcum = 48 grams of copper. However, this has to come with the caveat that in actual practice, the value could be anything up to an order of magnitude on either side of that.
|Coin||Copper content||Modern value (EUR€)||1st cent. Roman value (sestertii)|
|€2, €1||can’t calculate|
|50 c||6.94 g||3.76 c||0.145 HS|
|20 c||5.11 g||2.77 c||0.106 HS|
|10 c||3.65 g||1.98 c||0.0760 HS|
US coins (post-2009)
|Coin||Copper content||Modern value (USD$)||1st cent. Roman value (sestertii)|
|$1||6.24 g||3.78 c||0.130 HS|
|Susan B. Anthony dollar||7.43 g||4.50 c||0.155 HS|
|25 c||5.20 g||3.15 c||0.108 HS|
|10 c||2.08 g||1.26 c||0.0433 HS|
|5c||3.75 g||2.27 c||0.0781 HS|
|1c||negligible (97.5% zinc)|
UK coins (1997-2016)
|Coin||Copper content||Modern value (GBP£)||1st cent. Roman value (sestertii)|
|£1||6.65 g||3.15 p||0.139 HS|
|50 p||6.00 g||2.84 p||0.125 HS|
|20 p||3.75 g||1.77 p||0.0781 HS|
|10 p||4.88 g||2.31 p||0.102 HS|
New Zealand coins (post-2006)
|Coin||Copper content||Modern value (NZD$)||1st cent. Roman value (sestertii)|
|$2||9.20 g||8.55 c||0.192 HS|
|$1||7.36 g||6.83 c||0.153 HS|
I’m ignoring the value of the steel in the coins, because the modern price of steel is on the order of 1/10 that of copper. Also, I can't evaluate the 1€/2€ coins or the £2 coin, because they’re made of an outer ring and an inner ring, each made of a separate alloy, and I haven’t managed to track down figures on overall composition.
In term of practical value: it is impossible to make direct equations between ancient and modern currency because the goods traded are different, and where they are the same, they are generally used very differently. Donkeys and slaves were commonly traded in ancient Roman markets; not so much in a modern first world urban setting. Wine was cheap, wheat was expensive. Many goods that are standard commodities in modern markets simply didn’t exist (heating oil, coffee, cocoa ...). The Big Mac index has no meaning for antiquity.
What we can say is that a Roman infantryman was paid 900 sestertii per annum in that period, and on that scale, the sestertii prices that we see above for modern coins are ... not inconsiderable, actually. A tenth of a sestertius comes out to about 1/24 of a soldier’s daily wage.
On balance, it might well be fair to say that modern coins would after all have some value in an ancient market -- assuming the people you were selling them to (a) recognised the metal content of your coins, and (b) had access to a means for extracting the raw materials.