Mom & Dad will contribute the equivalent amount of an in-state public school - child is on their own for the rest
This was the system I wanted to use, but the problem is that the private schools are so much more expensive, that there's no way the kid can pay the difference.
For example, let's say we said "We'll pay $15,000 max." Since many private schools (e.g. Cornell, MIT, Washington U, Northwesten), cost around $40,000, there's no way the kid could pay the remaining $25,000 per year, unless he/she got major financial aid (which for us frugal savers isn't an option). There's no way for a kid to to get enough loans and jobs to pay $25,000 per year while studying.
So, practically speaking, what we'd really be saying is "You can't go to a college that costs more than about $20,000.
The cost of a college education is amazing - Why is it so high? What is driving the costs?
The way I figure it, the main cause is: Financial aid.
People spend all their money, get need-based financial aid, and the government and other big institutions pay for a good portion of the tuition. Thus they can afford an overpriced education, and the college can charge even more.
Imagine that tomorrow, all financial aid were eliminated. The colleges could no longer fill the seats at $40,000 a pop.