You're right, from an economics standpoint. However, there's a pretty hefty body of research talking about the effects of intrinsic/extrinsic motivation.
We've already seen merit pay fall flat in at least one instance ( http://blogs.ajc.com/get-schooled-blog/2010/09/21/another-bl... ), which suggests that economic incentives don't translate to better instruction or learning.
Additionally, Alfie Kohn's research (see his book "Punished by Rewards") demonstrates fairly conclusively that the addition of extrinsic motivating factors (such as money) tends to decrease intrinsic motivation.
I'd much rather see all the time and money that administrating a system like this would take go to a comprehensive professional development system, reduction of class size, and/or providing more opportunities for genuine learning beyond rote memorization, but that's just me.
(Edit: added quotation at the beginning for clarity)