The Good Shepard School was an example of an inexpensive (less than $400 a month) program that produced great results. They intermixed grades, and the older students taught the younger ones.
To save money, achiever type students (especially 8-13) can do lecture halls, watching movies, and taking notes. To save even more money, you can have computer rooms and libraries that are set up for students to act on their own. More students generally means lower cost per student.