I think there are reasons why cost would be nonlinear. First, there's simply demand. The people who want to do 60s clearly have a reason that 30s won't work, so

I think there are reasons why cost would be nonlinear. First, there's simply demand. The people who want to do 60s clearly have a reason that 30s won't work, so they may be willing to pay more (certainly they won't pay less). It's a different segmented market. There is a reason companies with lots of commercials tend to also be official sponsors of the Super Bowl. Second, practically it costs more. Ads are reshuffled around in real-time and the number of times you can be sure you can broadcast a 60 second spot are less than you being able to broadcast a 30 second spot, since the action may resume at an indeterminate time. Third, the Super Bowl specifically sells itself on the quality of the ads. It could do long term damage to the Super Bowl of the ads one year were just one company and not the funny celebrity heavy spots people expect.