Well, the Big East exit provisions are different. The problem for those schools wasn't the cost, it was the waiting period. But I think the Big East had a mechanism where you could pay more to get out more quickly. So there was wiggle room.
But yeah, Maryland will definitely try. Those are decent arguments Maryland could make to try to get the ACC to give them a discount, sure.
I'd have to read what the conference charter says but presumably Maryland agreed to be bound by any 75% vote to raise the exit fee. There's the meeting of the minds. Of course reasonableness is read into contracts, so there's an argument that $50M is just too high. But I don't think you'd ever see a judge invalidate the whole increase. It's a little harder to claim duress when you're a gigantic academic institution instead of some poor schmuck. And considering the fact that Maryland is exiting the ACC in order to make a whole lot more money, they're in a tough position to cry poverty.
You seem to know quite a bit about contract law for a non-lawyer.