This depends on many factors. Most smaller PCBs (100mm by 100mm or so) are not too expensive. The main cost of a low quantity PCB is the set up cost of the job, which can be as much as US$60 per job. The per square inch cost is usually about US$2 or so.
An excellent PCB fabrication house, such as http://www.apcircuits.com, charges about US$100 for two small PCBs with a turnaround time of 2 to 3 days.