When I picked up my ticket last week I asked about that and the Ticket Office said that around 70% of tickets was sold, so if that holds, I see around 19-21k sold tickets, so I see 17-19k at the game.
I cannot speak for the SEC, but I think for the MAC and the B1G, it is more of "there in the neighborhood" mentality more than anything else. In other words, a necessary evil for both sides....