Please point me to where it says CBS or Fox or NBC would increase the payout if they add more teams. It says they have agreed to amounts for the slots and content they are purchasing:

"Should it expand, the Big Ten would need a rightsholder (or rightsholders) to pay for its extra inventory. CBS, Fox and NBC already have their schedules set with what they believe to be cost certainty — number games to be televised, windows to televise those games and a "selection draft" for the most desirable games."

CBS Sports Chairman quote indicates they could expand to new slots:

"If there were additional games to be added in different time slots, we would certainly take a look at that. I'm sure NBC and Fox would also. Expansion, by and large, would only be a positive. But we would have to find windows and time slots to air any potential games but that would be a benefit to everybody."

Nothing there about them automatically paying more for an increase in teams + ESPN then taking the fourth slot. Seems clear that the increase in payout is related to additional additional teams filling additional slots that could be sold to somebody and the payout would increase. If that somebody is ESPN, the others aren't going to pay more just because new teams are added.