DaasYochid – Why would our Bais Yaakov & Yeshiva educated parents be unable to teach elementary school topics? Doesn’t that say something about our Mosdos?
Also, for your hypothetical parent with 5 children (@ 10K each), the parent that makes less would have to be making 80-90K, if not more, for it to be worthwhile to work over homeschooling. Already not most of the Olam.
To the point regarding fundraising, I agree that the money raised via that method should be segregated and used to pay for those who can not afford it on a needs basis. That means the amount of discount offered varies directly with the funds raised on a year over year basis, is limited, and does not mean that the Yeshiva takes all comers or offers a discount to all those who can not pay in full. That way, bills and employees still get paid.
As for marginal cost, we have been there, done that. As you point out, taken to its logical conclusion, no one should pay tuition.
To your point with CI & Gamanit, perhaps the most appropriate would be the opposite, to publicize those who do pay in full as “V’eilu Yaamdu Al HaBracha” (since you don’t want to call anyone a Off Tameh).
Chochom – fair enough. I don’t think Halacha (regarding what is an Ani) would agree, but there is certainly an emotional argument that can be made for keeping the present.