By my interpretation this is how the math works.
ESPN ACC contract adds new schools pro rata (Big 12 had this). I think the rate is $30M-40M. Let's say it's $40M for a round #.
By adding 3 schools, Stanford, Cal, and SMU. ESPN throws in $120M/year.
Last week it was expected SMU would forgo 100% and Stanford and Cal would forgo 30%.
If SMU and Stanford are now saying they could forgo all or nearly all of that for a number of years, the ACC can choose to re-distribute $80M among the existing schools to increase their payouts and cover travel costs.
If Cal forgoes 50%, we free up another $20M for redistribution.
So now $100M is available to go to the 15 existing schools = $6-7M / year each through 2036 --- or for as long as Stanford, SMU, and Cal are forgoing their full share. SMU signaled they might do it 5-7 years.
That may be enough to flip some (e.g. UNC, NCSU) votes to add some geographic-misfit mediocre football schools. But the risk is by adding more schools, you make it harder to disband the conference by majority vote which FSU and Clemson (+Maybe others) want to do.
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Big Ten -- does not have pro rata agreement. So while ESPN will automatically pony up new $ for new ACC adds, Fox will not. They'd have to value the new schools.
If Stanford comes in at $0 for the first few years and Cal comes in at $20M, there is no excess to redistribute to existing members. And when the entire deal goes up for renegotiation in 2030, the pie needs to grow, or Cal and Stanford are going to cut into the distributions of the existing teams.
This is how Fox's willingness to throw new $ out means they running that conference. The B10 Presidents want Cal & Stanford, but Fox needs to signal the pie will keep growing. Coming in for free/reduced cost helps in the short term, but may not solve the long term dynamic.