J.
I'm not familiar with the Chicago schools, but I know from experience that a lot of parents (even those who aren't "poor") get financial aid (or "scholarships") from schools.
But I think the going rate around here for montessori is more like 10-12,000 a year for the priciest. (I'm in Oak Park - we also have a progressive school, the The Children's School of Oak Park in Cicero, that's quite inexpensive. Not montessori, though.)
I think when we started in montessori, preschool age, the cost was only a little bit more than full-day daycare (our school had after-care that was pretty inexpensive, but the tuition was high for the actual school part of the day.) So the initial cost doesn't seem like that much and then you get used to the school and end up sticking with it. I really like the philosophy and methodology, but I don't think it's the only way to get an excellent education, either.