I know it was supposed to be a funny throwaway line, but I am irked by the "with $100 million you could interrupt the Super Bowl for 7 full minutes." That's not how sports advertising runs works. You are bidding on a limited amount of space determined by the game. I think there is also a non-linear cost.
Of course the NFL would never allow a 7 minute commercial break, although I do believe that the cost is linear. A 60 second commercial's cost is simply 2x 30 second commercials. There's no reason to do anything differently, since in the end it doesn't matter if that 60 seconds are filled by one or two commercials (aside from making the ad sales team's job slightly easier by having one less spot to fill).
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.