Apperently the highest paid athlete ever was a Roman Charioteer named Gaius Appuleius Diocles who made the equivalent of $15 Billion in prize money during his career. My first thought when I read this was Hah!, take that Michael Vick. ;D
I'd say it's somewhat different. Charioteers or other horse-based (my new word ;D) sports had to be, or were usually, rich. Chariot teams were a display of wealth by the owner. Football players don't have to be rich to succeed. They get rich after that.
Yeah, I'm thinking there must be some discrepancy as well. I think part of this may be due to the fact that chariot racing may have been the most popular sport within a society which had very few large-scale sports (not to mention entertainment options) to begin with. So this was probably like the NFL, NBA, MLB, and NHL combined.
The Chariot racers themselves did not have to be rich to race either, as you say, they were a display of wealth by sponsors. In principle no different than rich guy buying a football franchise or even a race horse and sponsoring a jockey today. I just find the amount of winnings this guy got amazing. Hell, I would race chariots for that kind of money, and chariot racing was very dangerous.