The advent of the salary cap ensures that under franchise models especially, there's always a limit to how much you can pay an athlete.
Indirectly, it results from a demand and supply equilibrium of the market but a salary cap ensures that there's a ceiling which can't be exceeded.
In professional football however, a player can earn as much as the market (or his employers) think he's worth.
I've looked at the main American sports as well as a few global ones. For individual sports based on prize money, I've simply considered the most recent highest earner (prize money unless contractual). All amounts are US Dollar equivalent and are annual amounts not representative of the entire contract.
Endorsements rightfully, have been completely ignored.