I believe the limit when investing in a child's education must be what the family can afford to pay. Nothing exaggerated like selling properties should be done. The most essential material in someone's education is love, followed by attention and care.
In case a child is a prodigy, he is doing great at school and show interest to knowledge, would you still not go over your limit to send your kind into most affordable college? What if the kid is next Nobel prize winner, or you feel like he is about to invent something, but he lacks some specific knowledge. Would you still send him to local or average school or university?