adding a school can bring money.

Bringing in schools that are good enough to compete for playoff spots can bring value from CFP payouts.
Bringing in schools that bring in eyeballs can bring value via TV money (not necessarily cable - looking at streaming services, etc. here)
Bringing in schools that are good enough to make high-profile matchups (e.g. the casual football fan will tune in for these).

IOW, it's not about market size, but it still is very much about a program's ability to generate revenue for the conference.