The ACC just reached a 15-year deal with ESPN. About 17MM per school, for all tiers, 1 through 3.
http://www.theacc.com/genrel/050912aaa.html
This is not going to hack it for FSU and Clemson. Well, it shouldn't anyway. If the ACC doesn't give them a huge disproportionate share, they'd be fools to stay.
Big 12 at 20, add 4 if they both come, add about 2 for the CCG, and they get to sell their own tier 3, which should be worth at least another 4. Plus, who knows how much more for better bowls, postseason playoff share, and higher ticket sales. (FSU's having problems selling tickets, which shouldn't be a surprise given who they have to host)
They'd get at least an extra 13 million for jumping to the Big 12. More than likely with the extra stuff I didn't put a number on, they'd double their money.