An important component of higher education costs and financial aid in the US is the "expected family contribution" (EFC) which is calculated based on information provided by student and/or family. An approximate relationship between family income and EFC for a family with one child in 2012 is shown below. Below about $30,000, the EFC is zero. By the time family income reaches about $225,000 the expected contribution is around $50,000 which was very roughly the total cost of attendance for the priciest colleges in the US in 2012.
To get a sense of how many families out there can afford what for higher education let's look at the distribution of family incomes in the US, also for 2012. The median family income is around $50,000 - in other words, about half of families are below and half above this level. The bottom 20% have annual incomes around $35-40,000; the top 20% have incomes of about $125,000 and above.
If we combine these two charts we can see what fraction of American families can afford (that is, would have "expected family contributions") various amounts for their children's higher education. The bottom 10% or so can "afford" zero, the next 10% a few hundred dollars. The second quintile of families (the next 20%) can pay maybe 1 to 4 thousand dollars. The median family would be expected to pay around six thousand. Sixty percent of families can afford $10,000 or less. And only when you reach something around the 75th percentile do you start seeing families whose expected contribution would be on the order of $20,000 which represents around a 50% discount on cost of attendance at an average private college and perhaps a 25% discount off the cost at a premier public college or university.
These numbers do not, of course, include financial aid, loans, and scholarships. The take-away is that based on expected family contribution calculations (which many consider to be extremely optimistic - that is, most families do not feel they can contribute as much as the calculation says they should) perhaps 10% of families can afford college "out of pocket" and perhaps 75% of families could not afford even the highly discounted cost of most private universities and colleges.
NOTE: The numbers here are simplified for the purposes of communicating the underlying concept/method. These comments focus on families sending children to college. The access and affordability questions, of course, go beyond this to include adults going to college full and part time at various points in their life. Thus, the considerations above are just a starting point for the conversation.
Goldrick-Rab, Sara. 2016. Paying the Price: College Costs, Financial Aid, and the Betrayal of the American Dream. Chicago: University of Chicago Press.