Rising tuition fees have gone hand in hand with a declining number of lower income students, so can be said as a fact that lower tuition fees increase the number of students from poorer families. A quick googling found this Education Dept publication noticing the trend http://www.studyinfinland.fi/tuition_and_scholarships/tuitio..., but I imagine more data can be found by looking at countries where tuition fees have risen faster than the income levels, like in Canada or USA.