http://www.seeedstudio.com/depot/fusion-pcb-service-p-835.html?cPath=185 also looks like a pretty good deal for boards. They're in pre-set sizes, and it's a minimum of 10, but 10 ea., 5x5cm boards are $10.
BatchPCB.com will do any quantity you want. $10 setup, then $2.50/in^2 with a minimum of one in^2 (but after that first in^2, they don't round up on the board size).