I don’t have the data for the most recent academic year, but the Record did a story way back in April 2002 on tuition and financial aid for the 2002-2003 academic year.
The financial aid policy makes it so that a family in the top quintile of family incomes will pay an average net price of $22,013 in 2001-02, or roughly 20 percent of its family income. A family in the bottom quintile, however, paid only an average of $1,683, or 11 percent of its family income. The difference between the net price and sticker price is made up for by grants from the College.
In other words, a family from the bottom family income quintile of American society was only charged an average net price of $1,683. Student loans and work study could then be offered to help the family afford that highly reduced figure.
Lest that picture look too rosy: Only 5% of students enrolled at Amherst, Carleton, Oberlin, Pomona, Swarthmore, Trinity, Wesleyan, and Williams in 2001-02 (only data I have handy) were from the bottom quintile. Indeed, only 28% of the student bodies of those schools came from the bottom 80% of American families sorted by family income.