For decades, the best-known college rankings have tried to encompass everything from "academic reputation" to dorm amenities. But a few years ago, we stripped all that away in favor of a simpler measurement:

Which schools give their graduates the best average return—in terms of salary—on their tuition investment?

With help from PayScale, a company that maintains salary profiles of 29 million workers, we collected median pay figures for two pools of each school's alumni: recent graduates (who have been out of school for an average of two years) and mid-career types (an average of 15 years out). For each group, we divided the median alumnus salary by tuition and fees (assuming they paid full price at then-current rates), averaged the results and, finally, converted that result to a percentage figure.

The outcome is a measure of return on investment that we call the Payback Score. For example, a hypothetical grad who spent $100,000 to attend college and now earns $150,000 a year would score 150. The higher the score, the better.

It's an imperfect bit of math. It doesn't take into account financial aid that students received. Neither does it include a host of intangible factors that can make any education more valuable or affect a person's salary prospects.

But a growing number of parents and applicants are giving such calculations more weight, in the face of a steady rise in college costs: Private-college tuition has increased 70% since 2001, to an average of more than $27,000 annually, according to the College Board; public tuition more than doubled over the same stretch. (Related article)

What's worse, a weak job market has made it harder for grads to pay off student loans. Steven Roy Goodman, a college admissions consultant in Washington, D.C., says that even the most prestigious schools have more questions to answer about their alumni incomes. "You have to look at the yield from your education," he says. "Not everybody who goes to Columbia becomes a multimillionaire."

Maybe not, but the average Columbia University grad can expect to earn $100,000 a year by her mid-30s, according to PayScale—2% more than a graduate of a top-priced public university.

So does that justify the extra $50,000 that he or she might pay for an Ivy League education, New York-style? You decide.

Here's the full transcript on America's best and worst college deals.

The Ivy League
Avg. payback score: 94
Avg. salary for recent grads: $55,463
Avg. salary for midcareer grads: $112,838

In any discussion of college expenses, the gold-plated cost of an Ivy League degree quickly becomes Topic A. Among this elite group of schools, four years of full-price tuition and fees for the class of 2009 averaged just under $138,000—82% more than what members of the class of 1996 paid. In response to this soaring bill, many of the schools have dramatically boosted financial-aid packages and offering grants to help families avoid heavy debt burdens.

Princeton, the top-scoring Ivy in our survey, was the first one to pledge to give grants, rather than loans, to all students qualifying for financial aid. At least 60% of Princeton students in the class of 2014 are receiving scholarships and other aid directly from the school, totaling around $40,000 a student. Harvard says the same is true for its incoming freshmen.

That kind of aid will make life easier for aspiring Yale grad David Marcano, a valedictorian and Eagle Scout from Worcester, Mass., who will be paying less than a tenth of the school's sticker price of $56,000 for annual tuition, room and board.

And if PayScale's numbers for future grads are any indication, he'll reap rewards afterwards. Ivy grads, on average, outearn their peers from the start, and the earnings gap between them and top-tier public-school graduates widens over time.

Still, critics say the Ivies have a long way to go to prove they're offering as much value as the best public colleges. Even at the schools with the most generous aid policies, some students get zilch, and many comfortable-but-not-rich families are facing five-figure student-loan bills.

For many, the biggest issue may be whether Ivy League degrees still carry the same economic clout in an age when many other schools have worked hard to improve their academic credentials and where liberal-arts degrees no longer necessarily pave the way to high-paying jobs.

In a study published earlier this year, economists Alan Krueger and Stacy Dale showed that students' SAT scores are a much bigger predictor of future salaries than the school they attend. Kids whose scores were good enough to enter elite colleges eventually earned as much as graduates of those schools—even if they went to college somewhere cheaper.

Non-Ivy Private
Avg. payback score: 79
Avg. salary for recent grads: $46,414
Avg. salary for midcareer grads: $94,181

It was a tough decision. Paul Ott says his son, Evan, tops in his class and a National Merit Scholar, got into a pair of well-regarded private schools and was offered some impressive grant money. But he found that even the private schools' best offers would leave Evan owing as much as $25,000 a year for tuition, room and board.

The Otts, who live in Dallas, are sending Evan to the University of Texas at Austin, where in-state valedictorians attend tuition-free their first year. "That was huge for us, because we're not well-heeled," says Mr. Ott.

Compared with the Ivies, experts say, most private schools have far fewer resources to tap for scholarships and grants. Indeed, the average Ivy League endowment was worth $10 billion at the end of 2010, while the endowment of the average college was worth only about $400 million. The schools in our study cost more to attend, on average, than the Ivies, and their tuition has grown faster since the 1990s.

The loudest complaints may ultimately come from grads whose salaries don't measure up to their college bills. At midcareer, graduates of the schools in this group averaged income of $94,000 a year—certainly respectable, but 16% less than Ivy grads were making and only 6% more than that earned by graduates of much cheaper public colleges. As for SmartMoney's Payback Score, this group averages a 79, lowest among our three categories.

Some education experts blame the income lag on these schools' strong identities as liberal-arts colleges, noting that those who earn the most in this job market tend to major in other subjects, like engineering or business. At Sarah Lawrence College, in Bronxville, N.Y., graduates often choose careers in education, public administration or social work, and come out earning, on average, just $38,600 after two years. Officials at Sarah Lawrence say that figure may underestimate alumni salaries, but also contend that's beside the point: "Their rewards are measured not just by earnings but by how much they are giving back to society," says Tom Blum, the vice president of administration.

Public Schools
Avg. payback score: 141
Avg. salary for recent grads: $48,486
Avg. salary for midcareer grads: $89,662

If our payback survey were a football game, the public schools would be spiking the ball in the end zone. Among recent grads, public alumni earn annual salaries equal to 52% of their four years of tuition and fees; private-school kids bring in just 32% on average. For out-of-state students, full-fare tuition at the schools in our survey is 35% lower than at private colleges, but in many cases, the career options are just as strong.

Ralph Mobley, director of career services at Georgia Tech, says 63% of the 2011 graduating class had jobs at commencement, up from 53% of the class of 2010; computer science and chemical engineering majors were among the hot recruits.

Public colleges have some troubles of their own. As tax revenue shriveled in the recession, many states slashed the budgets of public universities and raised tuition for out-of-state students.

From 1996 to 2009, the cost of a four-year nonresident degree at the schools we studied more than doubled, to $93,500. At UT-Austin, where out-of-state tuition has grown nearly 70% in five years, the average loan debt for undergraduates is now almost $25,000.

Still, the budget cuts haven't yet tarnished the public schools' bargain status. And for students willing to stay in state, the financial equation looks all the better. Mr. Ott, the Dallas father who counseled his son to go to UT, says they are anticipating getting an additional $2,000 or more in state scholarships from the university.