It's always a little bit dangerous to compare ancient economics to modern ones. But if one takes what a typical blue-collar worker (say, a butcher) makes in a year as the index, the second-century Roman charioteer Gaius Appuleius Diocles earned the equivalent of fifteen billion dollars over his twenty-four year career as a charioteer in Rome.
That's the equivalent of $625,000,000 per year. Compare that to contemporary athletes -- Cristiano Ronaldo (Real Madrid, £11,300,00, or in U.S. money, $17,459,630), Peyton Manning (looking to improve over his $98,000,000 six-year deal, making $14,000,000 of that this year), and Tiger Woods (took a purse of just over $10,500,000 last year). Granted, this doesn't include product endorsements, but it also doesn't include any other money that Diocles might have earned other than on the circus maximus, which is not recorded anywhere. I've got to think that Diocles got paid not just a salary but took a percentage of his team's winnings, and may have been a part-owner of his team. In that sense, the modern equivalent would be closer to a NASCAR team's star driver rather than a star performer like Manning or Ronaldo.
Consider also that a serious occupational hazard for the Roman charioteer was death, something that Ronaldo, Manning, and Woods are not at serious risk of being killed on the job. Manning maybe a little bit, but NFL players wear a lot of protective gear and the complex rules of NFL make a life-endangering injury for an American football player very unlikely -- only one player has ever died on the field and it wasn't from being hit. (Major league baseball has a similar mortality rate, and that single fatality was from actual play -- shortstop Raymond Johnson Chapman of the Cleveland Indians was hit in the head by an inside pitch from New York Yankees pitcher Carl Mays on August 16, 1920.) Soccer is a good deal more dangerous than many Americans give it credit for being (a bicycle kick looks like a really good way to break your own neck to me), but again, life-threatening injuries are actually quite rare. And golfers pretty much only risk being struck by lightning as a potentially lethal risk of play.
By comparison, Diocles would was at nearly constant risk of having his fragile, light chariot fly apart into debris and timber if he it a low spot on the racetrack; being ground into a smear of garum into the inner wall of the track; getting trampled by the horses should he lose his balance and fall; having a lead amulet or other object thrown at his head by a spectator and knocking him unconscious (in a foreshadowing of Philadelphia Eagles' fans' highly unsportsmanlike propensity for tossing D-cell batteries at opposing players or even their own players who fall into disrepute); or just plain stabbed by one of his a rival charioteers, who would typically carry daggers during the race for the explicit purpose of killing their adversaries.
Which makes me think of life insurance. In the modern world, an elite athlete gets compensation for the risk of death in the form of a life insurance policy, which is cheaper for his sponsors to buy rather than salary out of pocket. It only provides the compensation in the event that the risk actually manifests, but presumably would be a very significant amount of money, even compared with the athlete's annual salary. In the ancient world, there was something analogous to life insurance, in the form of a funeral club. Someone who had as much money as Diocles would have no need of a funeral club, of course; he or his lawyers would have set aside some money for a lavish funeral and monument, and he'd never have noticed the expense. But there wasn't life insurance then because no one had yet invented so sophisticated a financial product -- there's more to it than paying a premium now and getting a benefit later -- so perhaps the risk of serious injury or death could only be handled by such an excessive salary and profit-sharing.
So the lesson is that if you think our athletic heroes are overpaid today, be consoled that we're not nearly as far along that arc of decadence as the Romans.