Depends on how you do the accounting for the student athletes. For just scholarships, the cost is minimal. If you are kicking out a student paying full price to accommodate a student-athlete, then yes, it actually did cost SLU $50,000 in real money. Were there any paying students refused entry to SLU last year because of athletic scholarships? No. Did not happen.
The other way to look at it is to say how much does it actually cost SLU in real dollar terms. Looking at the dashboard from SLU https://www.slu.edu/provost/office-of-institutional-research/institutional-data/student-enrollment.php. Last fall there were 15,334 total students, 8669 undergrad and 6665 graduate students. If enrollment increased by 219.4 students, then how much does it really cost SLU? Maybe a professor or 2, maybe a handful of TA’s. This does not cost the university $11 million. Most classes have room for an additional student or two after all. Maybe some other increased incidental expenses. But it does not cost the university $11 million additional dollars to educate 220 more students. Another proof of this is that enrollment was up 130 students from the year before last. Did SLU go on a multi-million dollar hiring spree (they would have needed to spend $5 million more to cover the 130 additional students after all!)? No, there were layoffs, and the education of students went on.
The real issue this year is how much SLU will have to spend to pay for athletes. Using the example you gave above for IU, $20.5 mil cap, -75% for football, (which SLU does not have) leaves $5.125 Mil to match IU in spending total for athletes not on the football team. To keep the same proportions of IU’s spending across all other sports, we get $4.1 million for men’s basketball, and $1.025 million spread out among all other student athletes. Spend that much and we match IU dollar for dollar among non-football student athletes.