Read through this article: (also from 2013) $400.00 would not be inconsistent for multiple children in the districts mentioned there.
https://patch.com/illinois/grayslake/how-much-do-you-pay-for-school-registration-fees
I read the articles from 2013. It appears to still be the case in 2018-2019 that, while registration alone is not $400 (or even close to it), all of the extra costs (e.g., textbooks, extracurricular activities) can add up to that amount. And, of course, the more children you have, the costlier it gets.
Speaking with an attorney (one can be located through the Legal Aid link provided) is still my advice - but it might also be smart to contact the media (as suggested by Just Blue). The media can investigate and report on families struggling with registration costs.
And perhaps charitable organizations like the Salvation Army can be contacted for help.
There is not much sense in having fee waivers if those in need of a waiver cannot qualify for one.
Unbelievable.