I'll disagree with you as there are so many factors that need to be taken into account. It really comes down to individual situations in order to determine if private is the best investment you can make. I will say you certainly seldom can go wrong with private schooling.
As to the cost, I remember working with several people in Cincinnati who were ecstatic when their children would graduate as tuition at Ohio State was less than they were paying for high school. That always amazed me.