Money isn't an issue for the US equivalent of the top grandes ecoles - if you get in and are from a family with below median income you'll have a free ride for tuition, housing, and food. You have to be very well off to pay the full price of tuition these days. The real problems are that you're likely to go to a mediocre high school if you're poor, not have parents to help you out, have poor college admissions advisors, less time and money for extracurriculars, not be able to afford SAT prep classes, etc.  In other words similar to the issues that exist in France, though my impression is that the difference in public school quality between poor and wealthy neighbourhoods is much worse in the US than France.  On the other hand if you're non white you get the benefit of affirmative action which can compensate for some of this, though many minority kids who get in are from the upper middle classes, with many of the rest children of well educated but poor immigrants.