Here you can find some PCB factories you may need to prototype printed circuit boards.
We used this China-based service a few times, it is the cheapest for 2-layers small boards, you can get 10 5x5cm boards for 10$, and 14$ including standard shipping. Usually it takes a week for manufacturing, and 3 days (~35$) to 3 weeks (4$) for shipping. The quality is pretty good, down to 8mil trace/spacing (they say 6mil but there are reports of broken boards with these sizes), good finishing, 50% of boards are electrically tested.
This is the Sparkfun PCB manufacturing service. 2.5$ per square inch for 2-layers boards and 8$ per square inch for 4-layers boards, plus fixed 10$ handling. It takes 1-2 weeks for manufacturing and 3 days to 2 weeks for shipping. We used this service once, the quality is high, the best we got up to now.
US-based PCB paneling service, with domestic manufacturing. $5 per square inch for three copies of 2-layer boards, $10 for three 4-layers boards. We never tried this service, but it is said to be high quality. Nice purple finishing ;-)
This is another chinese manufacturer, we never tried it but it seems to be the cheapest one: $1.50 per square inch for 2-layers, $2 for 4-layers, $3 for 8-layers, with 100% electrical testing. Traces/spacing down to 5mil (standard), with optional 3mil upgrade. 2, 4, 6 and 8 layers.