Assuming Yale uses the IM calculation for financial aid, the problem isn't current income: it is savings and home equity. The financial aid formula actively penalizes savings in any form, as well as home ownership. If you have a meaningful nest egg in, say, the 1M+ range, you are guaranteed to have to pay full sticker price.
Therefore, in addition to being unemployed when my kids make it to college in 2035-36, I'll either have to blow our retirement funds before we turn 50, or persuade my kids that the Ivies just aren't worth it. Given my slacker tendencies, they'll pick up on the latter course of action via osmosis; never before have I rooted so much for nurture over nature!
Of course my wife won't let me try this at home.
(not really)
As far as bad plans go, it wasn't exceptionally terrible, because we actually had an exit plan. Wisconsin allows you to annul a marriage if you haven't consummated it after a year. (I might not be 100% correct on that point, but the law was something like that.)
Pretty glad I didn't go through with that plan.
So why not make the list price $100k? If your example is correct, this would only affect those who make between $250k and $500k (where 20% of their income is between $50k and $100k).
I don't think your analysis is quite correct. Assuming is $100K and they ask for 20%. If I make more than $500K, then I pay $100K. If I make $50K then I pay $10K.
But please note that I'm GREATLY simplifying things here. It's not simple "Give us 20%". If it was, it would be like PA income tax - one page - give us 4%. It's more like NY income tax - dozens of pages with lots of special cases and loopholes.
For the same reason I like PA taxes, I like Yale financial aid. It's closer to the PA model than to the NY model. I don't approve of thresholds like "Income less than $100K - you pay nothing". I could even argue that an Ivy is such a privilege that the "give us 20%" should have no upper bounds ;)
Very few other countries in the world have schools this prestigious, by the way, so it's not necessarily valid to compare them.
Edit: by the way, I highly doubt that there is any country in the world with universities that operate completely independently, with virtually no State oversight over budget, admissions decisions, curriculum, or degrees granted, and which can charge students however much they want, such that the State also pays the education bill for students.
My concern is with bringing in parental income to the situation.
We want to encourage the population to grow in sustainable ways, not only in some families that make having lots of kids a priority.
Now, the /parents/. They do have a choice. The general system outlined in Larry Nivin's science fiction universe where exponentially higher costs were associated with children past the first free one (there were also lotteries to fairly distribute most of the rest of the population sustenance quota, as well as the option of emigrating off of earth to unrestricted colonies).
Mostly, however, is the issue that today there are many who do not /have/ children because the imbalances in today's society make them feel unworthy or simply unable to do so.
The entitlement in this thread to ultra-elite luxury goods is insane.
Penn State wouldn't even give me a merit scholarship, regardless of my (rather stellar) high school academic credentials, community service and social work, and extracurriculars (worthy of Ivy League admissions), so I was looking at the full 25k/year sticker price. IIRC, their honors college had even more tuition fees than the regular college.
UPitt also gave me a meager financial aid package and initially refused to honor their own UPitt Science Fair merit scholarship that they awarded me while I was still in high school for winning first place (they insisted it was already included in the "federal grant money"-only financial aid package that they gave me, though it seemed suspect, it felt like they "forgot" about it entirely until I called the right people; THEN I got my scholarship included as additional financial aid).
Especially in the latter case, I'm not sure where the entitlement was; I won a merit scholarship awarded by the state school, and then the state school "forgot" about it and then tried to substitute it with federal grant money.
I went to the Ivy.
EDIT: Staying on topic, on the note of the expense of having multiple children: My sister went to the state school, and it effectively tripled the amount of money my parents and I were paying per year to go to school (and it put all four of us in debt; I helped my sister out because I graduated before she did and managed to land a great software engineering job right out of school with a starting salary significantly higher than my parents' income -- I'm still paying off student loans four years later now).