Hoooboy. Okay, it's not economics, purely, that drives undergraduate admissions; it's also historically been racism and religious bigotry. That, then, directly determines who the alumni are. The first quota systems were set in place in the Ivy League schools to reduce the number of Jews diluting the admissions pools. They weren't even embarassed about it:

"As the sociologist Jerome Karabel writes in “The Chosen” (Houghton Mifflin; $28), his remarkable history of the admissions process at Harvard, Yale, and Princeton, that meritocratic spirit soon led to a crisis. The enrollment of Jews began to rise dramatically.By 1922, they made up more than a fifth of Harvard’s freshman class. The administration and alumni were up in arms. Jews were thought to be sickly and grasping, grade-grubbing and insular. They displaced the sons of wealthy Wasp alumni, which did not bode well for fund-raising. A. Lawrence Lowell, Harvard’s president in the nineteen-twenties, stated flatly that too many Jews would destroy the school: “The summer hotel that is ruined by admitting Jews meets its fate . . . because they drive away the Gentiles, and then after the Gentiles have left, they leave also.”

The difficult part, however, was coming up with a way of keeping Jews out, because as a group they were academically superior to everyone else. Lowell’s first idea—a quota limiting Jews to fifteen per cent of the student body—was roundly criticized. Lowell tried restricting the number of scholarships given to Jewish students, and made an effort to bring in students from public schools in the West, where there were fewer Jews. Neither strategy worked. Finally, Lowell—and his counterparts at Yale and Princeton—realized that if a definition of merit based on academic prowess was leading to the wrong kind of student, the solution was to change the definition of merit. Karabel argues that it was at this moment that the history and nature of the Ivy League took a significant turn."

J. Karabel. The Chosen: The Hidden History of Admission and Exclusion at Harvard, Yale, and Princeton. Mariner Books, 2006. ISBN 061877355X.

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You can read about the methods of selecting for less academically gifted, but white and nominally Christian, students to your heart's content. But please know it has nothing to do with the economic ability of the child's parents to contribute to the school, nor of the child, as an alum to give back to the school in significant gifts. It's about shaping the "experience" of a Harvard, Princeton or Yale education. "In the nineteen-eighties, when Harvard was accused of enforcing a secret quota on Asian admissions, its defense was that once you adjusted for the preferences given to the children of alumni and for the preferences given to athletes, Asians really weren’t being discriminated against. But you could sense Harvard’s exasperation that the issue was being raised at all. If Harvard had too many Asians, it wouldn’t be Harvard, just as Harvard wouldn’t be Harvard with too many Jews or pansies* or parlor pinks or shy types or short people with big ears." Ibid.

*"Manliness" is big, too.