... at least for Colorado.  Within the last few days at leasts.  I recall reports that Fox doesn't have to pay it per the contract, but lots of people have been saying that they are going to pay it for the teams under discussion.

Dodd

It is expected that Colorado would receive an equal media rights share ($31.7 million, beginning in 2025) to join the Big 12, making the move even more attractive. The league's contract with ESPN and Fox is believed to guarantee an equal share for any expansion teams so long as they are currently Power Five members.

The Athletic

But when it comes to the Buffaloes, there’s not much to debate.

The Big 12 wants them. They’d have the support of ESPN and Fox as full-share members. There’s no early exit fee. There’s little doubt they’d be competitive and valuable for their former conference. 

I've heard it rumored that Fox will pay full share for any of the 4 Corners.