I think it really just comes down to marquee matchups. If ND joins the B1G, then you get a minimum of 3 more national games per year against tOSU, Michigan, USC, UCLA, and/or Penn State. Ranked games featuring ND are always going to be nationally televised with huge ratings, similar to Texas. That is far more valuable than Notre Dame vs. an ACC schedule, therefore Notre Dame is more valuable in the B1G.
In 2021, ND vs Ohio State had over 10 million viewers.
Notre Dame vs USC in 2022 had just under 7 million viewers.
the last Notre Dame vs Michigan game did over 7 million viewers.
Notre Dame vs. Penn State would likely do 6+ million.
Here is their viewership with an ACC leaning schedule. You could probably pencil in numbers like these against lower tier B1G competition like Indiana and Minnesota, which would also be more valuable than the million or so viewers those crappy teams usually pull.
Notice the biggest ratings draw.