BYU came in around 710,000 fans.
Other recent Big 12 additions:
Cincinnati - 323K
UCF - 507K
Houston - 100K
UU - 352K
Arizona - 406K
ASU - 606K
Colorado - 494K
Tell me again, Pac-10, how the school whose fan base was less than half the size of BYU's was the better choice for expansion?