Good point. I actually implied as much at the end of my previous post (#114), but for some reason it never occurred to me that since $9,800 is less than $13,755, government spending per student would go down if absolute spending remained constant. Duh.
Off the top of my head, there are a couple of ways to look at this. One is that increased spending on education is probably desirable one way or another, and vouchers might be a way to spend that money more efficiently (and would, incidentally, be a good way to get conservatives on board with significantly expanded school spending).
Alternately, you could keep spending the same. If you conceive of the state as merely the mechanism by which the people make collective decisions and take collective actions (and not necessarily as an entity separate from (and/or opposed to) civil society, then the important thing is that the total amount of money spent by society on education would remain constant; only the route the money takes would change. Of course, this would effectively represent a regressive tax, which is a tough nut. Most of the money would flow back into the economy (and much of it back into tax revenue), though, so it’s not a total loss.