The highest paid athlete of all time was a Roman Charioteer
When comparing across time periods as broad as this, pretty much the only thing that matters is how you compare purchasing power and adjust for inflation. If you compare something that hasn't gotten appreciably cheaper over time - say, gold, or skilled labor - you wind up with an extremely large number. Compare something that has - like grain or clothing - and you wind up with a much more reasonable number. Going based off agriculture, back-of-the-envelope math gives about 400k people in Rome and $1000/yr expenses on staple foods for about $400m, which is significantly less than the probably-gold-based conversion scheme the article's source uses.And if you want to be really persnickety, no amount of money in Rome would buy antibiotics, televisions, etc, etc.