Interestingly, it may be that one could pay twice as much per teacher and still end up with a lower cost per pupil overall, as administrator overhead plays a large part in the regular cost formulation. $125 / 30 students ~ $4k, a fraction of both the DC metro public and charter school cost per pupil (http://www.cato-at-liberty.org/2009/04/03/dc-vouchers-better...).
So, a base cost of $125K/30 kids. Add in benefits for the teacher and you're at $150K. Liability insurance is another $50K. Rent per year for 1500 sq ft to house the 30 kids (the classroom is smaller, but you also need to amortize the cost the space needed for admins, gyms, halls, locker rooms, cafeterias, etc) is at least $30K. Supplies (books, sports equipment, etc) conservatively cost about $10K for the 30 students. And, realistically, you'll always have some administrative overhead (tech guys, librarians, lawyers, principal, secretaries, etc) which I can't imagine being less than $50K (amortized over a sufficiently large school).
That brings our total cost to, at least, $290K - which is about 10K per student. Still not too bad (although I'm still probably missing/underestimating some expenses ...), but at least marginally more realistic than your made up $4K/student number.
I like the idea of school vouchers and private schools too. But you have to be realistic.
IIRC the nationwide average for public schools is about $9K/student, and that includes areas where the cost of living is a lot lower than it is in NYC. So if they can actually get six-figure-salary teachers and still keep costs down to $10K/student, that would be fantastic for the profession.
