They had no choice but to do it now. Everyone knew once they lost this weekend that he was going to be fired. It was a matter of when, not if.
Think about potential future coaches watching this play out. What if they let Frost twist in the wind for 3 weeks just to save a couple of bucks? A Nebraska legend like Frost? A guy who won them a NC as a player? If they do him dirty like that to save money, how are they going to treat someone like me if it doesn't work out?
They paid $7.5M for good public relations. Cost of doing business.