It seems to be similar at Yale, @TiggerDad. The amount contributed from the endowment as operating revenues ($1.2 billion) was substantially less than the nearly $3 billion in investment gains in the latest year for which we have financial statements. Comparing the two schools, though, shows the similarities and differences more clearly.
Princeton had $1.8 billion of operating revenues, of which $1.1 billion was contributed by the endowment. Yale’s operating revenues were roughly twice as large on a gross basis, at $3.7 billion. As noted above, a similar amount of $1.2 billion was contributed to Yale’s revenues by the endowment (Yale’s endowment is slightly larger than Princeton’s and their investment returns are similar, so this is what you’d expect).
In Yale’s $3.7 billion, though, is $905 million of medical services income, relating to the School of Medicine, the hospital and clinical practice organization, of which Princeton has no equivalents. Similarly, Yale’s $768 million of grant and contract income looks large relative to Princeton’s $296 million of government grants and contracts, but a lot of it comes from managed care companies, BlueCross BlueShield, Medicare, Medicaid, etc. In total, Yale reported just under $1.7 billion in revenues from the School of Medicine (of which only a small sliver was income from its endowment that would have been reported as part of the consolidated endowment income total). So, when you strip out the School of Medicine to compare the two schools more closely, they are more similar than you’d think, with Yale slightly larger in terms of budget and endowment.
That said, Yale has far more students. Princeton has about 5,200 undergraduates and about 2,750 graduate students; Yale, halfway through its expansion of the college by 15%, will soon have about 6,200 undergraduates, with about 2,900 graduate students and about 3,250 in the professional and arts schools (Divinity, Law, Art, Music, Forestry & Environmental Studies, Nursing, Drama, Architecture and Management), as well as about 850 in the School of Medicine (not including medical residents and PhD students). Accordingly, Princeton is far wealthier “per capita”, which is probably why they’re in a position to be planning an expansion of their own (https://campusplan.princeton.edu/).