Having listened to the teleconference, some telling details come out (which can be read in the linked PDF transcript). First, the ACC presidents have agreed to a exit fee that is 125% of the annual ACC budget (which is a 1/13 share of the annual TV contract) - so the $20M number thrown around is based on the current contract. Second, the ACC is able to renegotiate their TV contract with ESPN (but not shop it to other networks at this point) since they added two new teams. This may mean that the TV contract could become significantly better in the short-term as the 'value' of the new media markets are factored in by ESPN and that the exit fee will increase accordingly.
The ACC was in panic mode but by bringing in 2 new schools and being able to renegotiate the ESPN deal to account for the new markets and additionally tying the exit fee to the TV deal, the ACC has made it extremely difficult to pick off a member - assuming ESPN is willing to play ball on the renegotiation.