Some make a lot of money, some don't ... agent receives a commission that is usually between four and ten percent of .... A sport agent could make a percentage from each of several players
It depends which sport, how prestigous you are, how lucky you are. Usually you will not be paid as much as the star of a team but if you are sucessful you can make easily 1/2 to 1 m dollars.
http://en.wikipedia.org/wiki/Sports_agent