First, a table on income of people aged 25-34 based on sex and educational attainment, from the National Center for Education Statistics. The table shows a gap between incomes of college grads and high school diploma only (or GED) and the gap has increased over time. This itself doesn't show causality. One possible explanation is that college education raises human capital that has value in the labor market. Another explanation is that well paying blue collar work has been drying up. A third explanation is that educational attainment acts as a signal of worker ability which exists independent and prior to any cultivation of that ability by schools and that the return to ability has become more accentuated. The first and second explanation suggests that going to college is the path to higher income. The third explanation does not as under that explanation it is ability that determines incomes.
Now a couple of pieces on the issue of whether there are not enough low-income students in college. Here is one such article from the Christian Science Monitor. (Amherst College, featured in Declining by Degrees, is discussed in this piece along with other institutions.) It makes an interesting point that financial aide is only part of the issue. Another part pertains to information flows and culture. First generation college students from low income homes have to be actively recruited, because they are not otherwise exposed to the right information to become aware of the opportunities available to them. I'm not suggesting that you read this book, but The Gatekeepers by Jacques Steinberg does offer a detailed look at how this recruiting process works from the point of view of a particular recruiting officer at Wesleyan University going after an Hispanic student of modest family income and educational attainment. Another article, this one from Inside Higher Ed, makes a similar point about family income of college students (those families are richer than the population as a whole and the gap has gone up in recent years). There is also some evidence in that piece about the racial composition of college students over time.
These articles are getting their data from the HERI Survey of Freshmen. You might find the PowerPoint that summarizes their results interesting. The survey has been going on for 40 years so it is much better able than NSSE to provide a look at trends.