The Koch brothers spent roughly $300 million during the 2014 election cycle and came away with control of the Senate and a stronger grip on the House. Pretty cheap investment when you're worth over $85 billion (their combined net worth). Great ROI, too.
By way of comparison, the average NFL franchise is worth $1.43 billion. If they wanted to buy the Dallas Cowboys, it would cost them upwards of $3.2 billion. And what would they get for their money? Nothing but frustration if the team didn't do well. Then there's those players always wanting more and more money. Who needs that?
Buying Congress was so much cheaper and promises to be much more fun to own.