When the Big 12 was at risk of falling apart, ESPN stepped up and said they would continue to pay the 10 teams as they used to get with 12 teams, including the championship game money. As you point out, the Big 12 is now being paid under a new 10 year contract -- but they are no longer being paid as if they have 12 teams. Their issue with expansion is that no expansion candidates, unless they steal teams from a P5 league, will add enough value to increase the TV contract -- even with the added money of a championship game. While the new 10 team deal is fairly good, it is really no better, per team, than what other conferences got who renegotiated at the same time -- and is going to be worse than what the Big 10 will likely negotiate in the next couple of years.

From what I've heard, BYU is on the borderline of adding enough money but no second team provides enough added money to increase money for the conference.