It all depends on the private school. We sent my daughter to a very small (and relatively inexpensive) private school because the public schools in the area had a lot of issues (poor education, drugs, etc). When she was in 8th grade, we moved to a different county that had some tremendous private schools. But by that point, she didn't want to change schools because of her friends. So we just let her stay at the private school. However, if she had gone to the public alternative, she could have almost knocked out a year of college credits.
But for the really great private schools in our area, you're looking at around $30,000 plus per year. The most widely recognized one has tuition of $23,000 for Kindergarten. But yeah...the education is fantastic.