For most private schools (and the more prestigious public schools), the only people who pay list price are those most able to pay it. Working class and even squarely middle-class kids will generally qualify for significant amounts of need-based financial aid in the forms of grants. So, to the degree that cost is a factor, you should be considering the tuition in conjunction with the financial aid package offered.
I think neither cost (as long as the cost is affordable) nor prestige should be the top criteria.
I think its more important to find a good match between the student and the environment of the college. The most important aspects of the environment are the professors, the other students and the available academic programs.
I'd discount big research universities, since the foremost criteria used by the institution for evaluating undergrad instructors is 1) are they cheap (ie grad students), 2) does their research attract prestige, funding, and cheap labor (ie grad students). The foremost criteria used by undegrads in evaluating their profs is "coolness" and the ease of getting a good grade. Also, access by undergrads to profs is limited. They are usually teach in big lecture environments, and their office hours are limited.
Focus more on institutions, like small liberal arts colleges, that place a high value on undergrad instruction, while still expecting profs to maintain a level of engagement in scholarship and research. The classroom environment is usually more intimate and profs are more available outside of the classroom.
The students are important too. Every hour spent with a prof will also be spent with other students. Better they contribute to the experience. Also, for every hour spent in a classroom setting, there should be at least as many spent studying and socializing with other students.
Even if you have your criteria firm in your mind, its hard to get enough information to base an evaluation on, so it pays to look for points of leverage among friends, family, family doctor, high school teachers, etc. None of them are going to have good knowledge of more than one or two institutions, but teachers will have a sense of the experiences of former students. Older friends and relatives will know about their own institutions. Parents of friends and relatives will know something about the experience of their own children, and also of the children of their own friends and family. What's most important though is that these people will have an inside perspective not just the institutions, but on the prospective student. So, tell these people what the student is looking for, ask them for suggestions based on what they know, and then ask them why they made the suggestion they did and see if it makes sense.
It's also worth paying close attention to the way the schools markets themselves. If a college manages to communicate a strong and consistent identity for itself in its marketing materials then i'd give that identity some weight unless their is strong evidence to the contrary (particularly if the same message is delivered once they've issued their acceptance). There are some perverse incentives for small schools to misrepresent themselves, but these mostly come into play before applications are received. Also I think these perverse incentives are checked by the fact that they don't have a lot of flexibility with respect to class size. Too many students and their quality of teaching suffers. Too few and their finances suffer. In addition, if its a bad match and the student drops out, it can damage their reputation, and hurt some of the metrics on which they are judged when it comes time for re-accreditation.
Take advantage of the net to get more of an inside view on schools too. We used to have prospective students checking out our live journal group. I think that's died off a bit, but there are still groups on facebook and whatnot, and the school should be able to put you in touch with current students and recent grads.
In the end though, its great to do a campus visit, particularly if you are choosing among just a handful of schools. Arrange it with admissions. They may get you a room for a night in a dorm, and they'll get you into classes, and give you a campus tour. Outside the formal activities, take advantage of unscheduled time to talk to other students.
Regarding the academic program, a lot of students end up graduating in something they might not even have known about when they entered, and they may end up making a living doing a job that didn't even exist when they graduated. For these reasons, I think its important to get a good solid liberal education, with exposure to a variety of subjects and viewpoints no matter what major they choose, and a grounding in the humanities, so they have a perspective on the forces that have shaped society in the past, and can apply that perspective to understanding present day changes.
Last thing, I think the prospect of launching a startup after graduating was raised in another thread. I generally encourage people to keep their options open after graduation by not taking on too much debt. For a student who is planning on a pursuit (like writing a book, or starting a company) where they can expect little or no income for a long stretch after graduation, I think the goal should be graduating with no debt at all. In those cases, cost could become a more important factor; 10K in loans might not be decisive if one expects to have some sort of paying job after graduation, but it could be the difference between having grocery money, or going to bed hungry if you are planning on scraping by while trying to get a startup off the ground.