At the bottom of the scale in terms of fees are the parochial schools. The Roman Catholic, Jewish, Episcopalian, Lutheran, Seventh Day Adventist, Christian and Muslim schools offer great educations for a very reasonable cost. Most of these schools are day schools. Schools like the Cristo del Rey schools specifically target at risk children from families which could not otherwise afford a private school education.
Helping the affordability factor is the initiative taken by several leading prep schools. Read Private School May Be Free If You Make Less Than $75,000 for details.
Remember: most private schools offer generous financial aid. Be sure to ask each school on your list. Don't forget to apply as early as you can. Don't wait until the last minute if you are looking for financial aid.