What about vouchers and schools of choice?

I recently read several publications by John Chubb.  He is in favor of schools of choice and school vouchers.  I understand the basic rational, but could not see where the cost of the programs have been sufficiently weighed.  Funding is typically allocated to school districts per ADM.  With Charter Schools, and voucher programs, all of the ADM funding per child is allocated to the school in which the child attends.  This seems fair, right?  What if the school district would not have allocated the same amount per child to a particular grade-level.  Does it cost the same amount to educate children at each grade-level?  If not, where is the equity in the allocation of funds between schools?  It just seemed like an interesting way to consider schools of choice.