Plenty. Voucher deals typically include the tuition ceiling and non-discrimination clauses. The latter is hard to enforce, so it is not much of a concession by the vendors.
The tuition ceiling is not necessarily onerous either. The average spending per pupil in a district -- a common metric for setting the voucher level -- obscures significant variation in spending by school. With the right selection of pupils, it is not be hard for a vendor to make money. Aside from their ability to skew their student body, the vendors have a key cost advantage -- they can start a new school with new teachers earning entry-level pay. The public schools are obliged to retain their current workforce, whose pay varies significantly. Labor costs are about 80% of school costs.
I discuss this in my thrilling book, "Risky Business: Private Management of Public Schools," co-authored with Craig Richards and Rima Shore. (It will change your life.)
mbs