My college did a lot of good for me, and the staff I interacted with seemed to be working below-market because they believed in what they were doing. It's a non-profit institution so I expect they would spend donations on something that they thought (rightly or wrongly) furthered education rather than just giving them to someone. And what I paid at the time was subsidised by previous donors and the government, at a time when I couldn't've afforded to pay much more; now that I've done well in life, in a way that seems at least partly off the back of that education, it seems fair to contribute something back.