They already do bring this argument into healthcare.
Some medicines literally do not get prescribed because they are prohibitively expensive: they work, but it is deemed that the benefit derived from using them isn't enough to outweigh their cost.
I'm not saying disabled children don't have a right to an education but if it was going to cost 500,000 per year would that be reasonable? Why is 100k reasonable if 500k is not? Where do you draw the line?
It cannot only be about the benefit to the child, sadly. Society is based on shared costs for mutual benefit.
Nobody ever wants to think about this stuff because yes it involves some really tough decisions but maybe we just cannot afford to give every disabled child their optimum education, just like the 8k spent on all the other state educated children does not deliver anything like an optimum education for many of them.