The tithing that comes out of the alumni alone would be one thing. But far above and beyond that, the leadership training and the commitment of those who come out of BYU is very much worth the Church's investment.

That is a temporal assessment. Here's a spiritual one: Education is an absolutely indispensable part of the doctrine of the Church. Go back an look at the history of this Church with regards to education and you will have the answer to the question you are asking. We have to be in the education business, and not just spiritual education--for all things are ultimately spiritual in heaven. We believe that Temples are schools of eternity and that schools are temples of learning, because it is all a part of the great process of growth, development, and progress in God's kingdom. Boyd K. Packer once spoke about this at BYU:

"There are two opposing convictions in the university environment. On the one hand, 'seeing is believing.' On the other, 'believing is seeing.' Both are true! Each in its place. The combining of the two individually or institutionally is the challenge of life. Neither influence will easily surrender to the other. They may function for a time under some sort of a truce, but the subtle discord is ever present.

"They mix the way oil and water mix—only with constant shaking or stirring. When the stirring stops, they separate again. It takes a catalytic process to blend them. This requires the introduction of a third ingredient, a catalyst, which itself remains unchanged in the blending process.

"Each of us must accommodate the mixture of reason and revelation in our lives. The gospel not only permits but requires it. An individual who concentrates on either side solely and alone will lose both balance and perspective. History confirms that the university environment always favors reason, and the workings of the Spirit are made to feel uncomfortable. I know of no examples to the contrary.

"Spirituality, while consummately strong, reacts to very delicate changes in its environment. To have it present at all and to keep it in some degree of purity requires a commitment and a watch-care that can admit to no embarrassment when compared with what the scholarly world is about.

"The moral and spiritual capacity of the faculty and what they shall give, and the spiritual atmosphere in which students are to learn and what they receive, will not emerge spontaneously! They happen only if they are caused to happen and thereafter maintained with unwavering determination. We at BYU can be competent in both and also merit the respect of those charged with the accreditation of institutions of higher learning. . . .

"I spoke of the catalytic process where two seemingly antagonistic influences can merge and each give strength to the other. The essential catalyst for the fusion of reason and revelation in both student and faculty is the Spirit of Christ. He is “the true light that lighteth every man that cometh into the world” (D&C 93:2). The blending medium is the Holy Ghost, which is conferred upon every member of the Church as a gift.

"The blending of opposites is everywhere present in life. A base metal, fused with a precious one, can produce an alloy stronger and more resilient than either component alone.

"On the one hand is reason: the thinking, the figuring things out, the research, the pure joy of discovery, and the academic degrees man bestows to honor that process. On the other is revelation, with the very private and very personal, the very individual, confirmation of truth. The combining of them is the test of mortal life!"

https://speeches.byu.edu/talks/boyd-k-packer_say-unto-one/