Bob Luddy, who started the Franklin Academy charter school in Raleigh, founded a series of private schools, Thales Academy. The tuition was $5K a few years ago, and I believe his costs are in the $6,500 range. Franklin Academy averages 1 year and 4 months of achievement for every single academic year on the areas measured by the Iowa Test of Basic Skills.
Also, Cheyenne Mountain Charter Academy gets about $6,800 per child to educate their children, and has amazing results (check out their CSAP scores online). Their high school was number 1 in the state in 2007.
If you really just want to educate kids, it's not nearly as expensive as if you have other goals in mind.