Big 12 made a lot more money... because they had (by far) the two most valuable programs (Texas + Oklahoma >> USC + UCLA), but current contracts aren't going to be in play as much.
My take is that the TV execs will decide
If ESPN tells the PAC12 they'll get $X M if they add Oklahoma St, BYU, Texas Tech and Kansas
And ESPN tells the Big 12 they'll get $Y M if they add Arizona, Arizona St, Colorado and Utah
If X > Y then the most desirable Big 12 teams will go to the PAC12. If Y > X, it'll be the other way around