I hear what you're saying, hooper. You are right, we do not have infinite resources. Especially now, with the defeat of 1098, we might be looking at making a lot of pretty cruel choices soon.
I do believe your numbers are off a bit. $10,000 per student per year is more like it for Washington State public schools, though admittedly that's averaged out to include all students, including the costliest (i.e., most challenged) ones as well as the most gifted ones, who also draw a higher-than-average amount of teaching resources.
Conservative source: http://tinyurl.com/wa-school-spending
Anyway, I think the cost of educating a challenged kid is perceived by society in much the same way as providing medical care for someone with a serious health problem. It's not something people generally view in terms of "Are we getting a good value for our money?"
You might say that healthy insured people "finance" the care of sick and uninsured ones, just as low-maintenance students finance the high-maintenance ones. That's how the education system got the way it is, with more money spent on some kids and less on others. I think people are generally ok with that system, even when money's a little tight.
Of course, this is not an iron law or anything, but it is worth taking into consideration whenever you try to tackle social problems like how to best fund medical care or education.
As Jan pointed out, you didn't answer my question about what you'd do if you had a challenged kid in public school yourself . . .