You're talking different top dollar amounts. The Big 12 is talking adding people between $30-40, adding to the SEC is twice as much. So top dollar is relative to what the Pac will receive vs what they'll make matching in the Big 12. And the reason they continued their current contract without losing money with Texas/Oklahoma leaving was two fold: One they were set to make a massive jump in money in the next contract, likely closer to $60m a team, so they did lose considerable value. However, two, over the past 5 years the remaining teams have increased their engagement with fans by 40% when many other groups are sinking. That doesn't make Kansas State equal to Oklahoma, but it makes them worth a lot more than they were worth the last time the Big 12 signed a contract.
As for the SEC, the issue isn't that Texas doesn't get top dollar for the games, it is that going to 9 puts more games on lower networks like ESPN2 and not ESPN/ABC. Those aren't worth as much and it means less games on the big networks for the lower brands of the SEC, who also get a vote.
They'll end up going to 9, especially after Texas/Oklahoma get a vote, and they'll get more for it, but the SEC is trying to do it mid contract, as opposed to the B1G who did it at the end of a contract, that's part of the reason too.