Charter schools are fine by me–but they are not an educational panacea. They may do slightly better than a public school that takes “all comers” in the same area, but part of that is almost certainly due to a higher level of interest in education on the part of the parents who move their children into charter schools.
As far as the cost per pupil goes, the pdf document here:
http://www.michigan.gov/documents/PSAQA_54517_7.pdf
indicates that the charter schools in Michigan receive the per pupil base foundation funding through the State School Aid Act, and that it may not exceed the funding of the public schools in the same geographical region. Given that the vast majority of public schools in Michigan are at the “minimum” per pupil level and that a large number of the rest are between “minimum” and “basic” funding (see my earlier post for the numbers and dollar figures), it seems improbable to me that In Michigan the charter schools are educating students for half the cost per pupil.