I think the underlying premise is that, of all the expansion candidates being discussed, BYU will bring the most value to ESPN's B12 deal. Compared to adding an inferior candidate to the B12 who doesn't currently have a separate deal with ESPN, but still having to pay full freight for that new, inferior member, it ends up saving ESPN money because ESPN gets the best possible expansion candidate for their B12 deal (BYU), AND they no longer have to pay BYU separately.

The alternative for ESPN is to overpay for an inferior candidate in the B12 deal AND still pay BYU on its separate contract.

This logic blows up if (1) another candidate would bring as much value to ESPN's B12 deal, and (2) ESPN is getting better than market value out of its separate deal with BYU.