My brother and I had this very discussion recently. Consider;
ND undergrad enrollment - 8,600
ND Tuition - $55K
Total Undergrad Tuition - $473M
The endowment would need to earn 3.6% to cover all tuition.
Why not just charge Room & Board and make tuition free? Or, means test the cost for attendees and dramatically lower tuition.
Of course this is overly simplistic and the endowment is used to fund a myriad of programs, but it's a discussion that has merit.