If Brey truly makes $800k and a $700k bonus how could he not at least pick up the phone for the job. Let's say hes offered $2.5 M a year (conservatively) that is a million more than he currently makes. In terms of fit, Notre Dame and U of I are both prestigious academic universities that probably have the same issues and selling points to recruits.
Stay in the Midwest, coach up a good recruitiing class (if they stay) and get some more cash. I know he has been at Notre Dame for quite some time but it doesn't seem out of this world crazy if the money is right.
Personally, I think there is absolutely no way that Mike Brey leaves ND for Illinois and actually think money will have nothing to do with it. Career-wise, it makes zero sense IMO. But even if money mattered, I doubt Whitman would turn this into a bidding war with ND.