What began as a casual lunchtime conversation in the office, turned out to be a very interesting question with surprising answers: do you make more money per month than a slave in ancient Rome? Although at first glance it sounds like an oxymoron, the question is indeed valid, since a slave has a cost, not only to buy, but also to maintain. It needs to eat, sleep and keep warm, otherwise he or she would not perform well or could even die, causing additional expanses for the master. So you might say a slave did not receive a salary, but more of an in-kind compensation in form of food, clothing and shelter. The fact that this sustenance is not in a form of a direct monetary compensation is just a matter of semantics, it is still a salary. Without any other option, he or she sells his or her own labor in exchange for sustenance, as miserable as it may be.
In order to reach a monetary equivalent we need to compute the maintenance cost of a Roman slave on a monthly basis. This is a complex task, so let us first set a frame of reference:
The most common units of measurement of weight in ancient Rome were the modius and the libra (Roman pound): 1 Modius = 20 libra = 20 x 322.5 gram = 6.5 kg.
The roman currency consisted of several units: 1 Denarius = 4 Sesterces = 16 Asses.
Now we can speak about the Roman prices and wages. The most common reference is the “Diocletian’s Edict of Maximum Prices” from year 301AD, which was issued to fight debasing of coinage and inflation. Before the inflation, judging by the prices of grain or remuneration of soldiers, the period of the early 1st century AD had a price level roughly a factor of 10 smaller  and amounted in the Roman provinces to something like this:
Food / amount / price:
- Wheat, 1 modius: 12-16 asses
- Barley, 1 modius: 10 asses
- Oats: 1 modius: 5 asses
- Cabbage, 10 heads: 2 asses
- Beef, 1 libra: 1 as
- Fish, 1 libra: 2-4 asses
- Chicken, 1 libra: 10 asses
Clothing / amount / price
- Tunika, 1 piece: 4 denari
- Cloak, 1 piece: 5-20 denari
- Boots, 1 pair: 2 denari
- Sandals, 1 pair: 1 denarius
- Unskilled laborers: 8-10 denari
- Skilled laborers: 10-25 denari
- Legionary (private): 20 denari
- Legionary (centurion): 300 denari
The first thing that one notices is that if an unskilled man wanted to make something of himself and there was no family fortune to boost him, he should join the army. So we definitely made some progress there – nowadays joining the army is not the only way to advance up the social ladder, though it still helps. Bad luck in that regard if you were born a woman, though.
An average Roman ate about 2 pounds of bread (i.e. wheat) a day, or 3 modii a month. With a price of say 12 asses per modius, together with a much smaller amount of fruit, vegetables and meat, the price amounts to about 48 asses per month. Slave’s clothing succumbs to wear and tear and has to be replaced adding some 8 asses per month. Finally, housing also has some cost and due to poor living conditions one would expect for a slave let us give it simply another 8 asses per month, taking us to a grand total of 64 asses or 16 sesterces or 4 denari per month. Looking back at the average wages of unskilled workers the number checks out, since one would expect slave labor to be cheaper than hiring a free man.
A quick internet search for Roman GDP for the early 1st century AD reveals a Wikipedia article  where different sources quote 166 to 380 sesterces per capita per year. Now the calculus is easy: Roman slave yearly “wage” = 16 sesterces/month * 12 months = 192 sesterces = 50% of GDP per capita, taking the highest estimate.
Now we jump some 2000 years to the modern age: German 2018 minimum yearly wage = 8.8 EUR/hour * 176 hours/month * 12 months = 18 670 EUR = 43% of GDP per capita, taking the IMF 2018 estimate . Even if we include the employer’s expanses for benefits, we end up around 50%. For the US the situation is even grimmer: in 2018, depending on the state, the minimum wage varied from 7 to 12 $/hour. If we take an average value of 9 $/hour, which also happens to be roughly the median wage of low-paying occupations , it corresponds to some 19 000 $/year = 30% of GDP per capita, again taking the IMF 2018 estimate.
When discussing how wealth is distributed, politicians often use the “pie” metaphor to describe how everybody should or should not get a slice equal in size. So if you happen to practice one of the “low-paying occupations”, chances are, slaves in the ancient Rome were getting a bigger slice of the pie than you are. This result is of course just a quick and dirty ball park figure and a factor of two could easily hide somewhere in there. Nevertheless it has come as a surprise to me, since it suggests that an ancient slave costs in relative terms about the same as a modern-day low-wage worker. With automation and hyper-production, one would expect that this ratio tipped in favor of modern workers long time ago, by a factor of 10 or even a 100. But in fact, it seems to have become worse.
 Money and prices in the early Roman empire, D. Kessler and P. Temin, MIT, Department of Economics, 2005
 Roman economy § Gross domestic product, Wikipedia, February 2020
 “National Occupational Employment and Wage Estimates United States”. US Bureau of Labor Statistics. May 2006. Retrieved October 6, 2013.