It's possible that the causation runs the other way: the parents are far more interested in a good education, so they're prepared to shell out the big bucks. And, because they're more interested, they're more likely to encourage, cajole, bribe, etc., their kids into learning more. Also, there might be a genetic factor: parents who are interested in their kids being educated (i.e. think that education is a good thing) are more likely to have children who think that education is a good thing.