After a little research, looks like the local PreK-6 Baptist school is a little under $3400/yr for first through sixth grades. (Less than half the ISD's annual revenues/enrollment.) They'll prorate that for people taking their kids out in full month increments, so effectively $340/mo. The nondenominational one is $3,950-4,400 depending on grade, with the same prorating, and goes through 7th grade.
As I understand it, interest beyond that was low, since the parents want to homeschool and/or put them into public schools once they have that 6th or 7th grade foundation. Frankly, the homeschooled kids I knew in HS were pretty much just there to fill out the assignments and get the diploma, because they were already beyond even the honors classes, but figured a genuine SHS diploma would head off potential issues later.