Basically, food is the marginal cost now. Books are

electronic now in large part. I'm using my freshman daughter at Clemson for that data point, but I suspect ND is the same. She didn't have a single physical book freshman year. In most cases, she didn't pay to have an electronic copy either -- although she likely was supposed to. As an aside, you also can't sell or buy used books anymore either because of this -- which is a negative to some extent and of course if you prefer paper you have to print.

Food is the hard cost ND has per body they let go free. Even there, they cook more food per day than people eat -- allowing for some margin per meal. Thus, you could argue that short of adding 100's of extra students, even food is not really a true incremental cost.

The infrastructure, doorms, electriity, etc. is all a constant whether those extra 18 hypothetical people do or don't exist.

Now, to invent a sport and a roster that never existed before out of thin air there are other costs outside of the student cost -- coaches, travel, etc. Those are incremental costs. However, this thread seems to be debating the mythical cost of a scholarship to a college. The reality is the education, room and board part is essentially zero for scholarship athletes. That same money was being spent with or without them.