Generally, it is prohibitively expensive. I come from a wealthy suburb where the private schools are a bit worse than the public ones, so it worked out for myself and my siblings as our parents could not have remotely afforded to send us to private schools. So far we've all gone on to great schools and careers after high school, so it's not always such a bad thing to be in the public school system, despite the OP's blanket statement.

EDIT: Here's a link to some older costs: http://nces.ed.gov/programs/digest/d10/tables/dt10_063.asp

$15,000-20,000 a year. They don't go to private school either. It's only marginally better option than the government schools.