Fairfax County, VA. It's one of the wealthiest counties in the US, so there's plenty of discretionary income for private school. But, I wouldn't have guessed other areas had tuition less than average spending per pupil at the local public schools.

I suppose there is always going to be a 'elite minimum' so that the precious child won't have to deal with the riff-raff. Very much like unpaid internships often become.