Well in Major League Baseball they don't have silly things like a salary cap. That's why the rich teams have payrolls of over US$120 miilion and the poor teams try and survive on US$25-30 million.
So the "Commissioner" (the MLB'S answer to Andy D at the AFL) bought in the this thing called a luxury tax which means the higher the payroll the greater amount of tax you pay to support the poorer teams. There's a catch though - all MLB teams are privately owned - and a number of the owners of poorer teams spend less so they can get more luxury tax but they put it in their own pocket rather into their teams.
After so many years (I think it's 7) the players can file for free agency and sell themselves to the highest bidder. As supporters it can break your heart - just as it can when the Clubs trade players half way through a season.
It is the side of baseball I don't like but in the MLB they cannot bring in a salary cap because of the existing contracts in place. The highest paid MLB player is 4 years into a 10 year US$252 million contract