That doesn't say it increases simply because teams are added, which was your premise above.

I have a hard time believing the contract says they pay more to broadcast the same number of games just because the pool of candidates increased.

Why would CBS, Fox, and NBC pay for more for the same # of broadcasts? All articles say they have their contracted slot. Unless there's an option to show another game on another platform (e.g. streaming) during that same slot, there's no benefit to having more candidates — plus they'd then be competing with themselves, anyway.

Contract isn't public, but I'm assuming they have first right of refusal if the Big 10 expands and now has more content to purchase another slot (e.g. the 10:30 ET slot). If they do, the contract increases. If they don't, someone else purchases it. Those two are likely mutually exclusive and they aren't going to be getting money simply to add more + able to go to ESPN for the slot as you suggested.