This post is about PCB (printed circuit board) services and full-fledged PCB manufacturing. You can etch your own one sided board (double sided boards are possible. There's a tutorial by BlondiHacks showing you how), or you can purchase them from a PCB service. Here's a list of PCB services and some advice on making your own PCB's. I will use a 2"x3", 2-layer pcb for my pricing comparisons.
Fusion PCB Service from seeedstudio: They have a minimum quantity of 10 boards, and use cm instead of inches. A 5cm x 10cm (50 cm^2 opposed to 38.71 cm^2 for a 2"x3" board) PCB costs $2.39 per board.
OSH Park: Formerly DorkBot, this Open Source Hardware company offers 2-layer PCB's for $5 per square inch for a quantity of 3. That means it's $10 per board for a 2"x3" board (you have to order in batches of 3, so you get three 2"x3" boards for $30). If you order over 150 square inches, it's $1 per square inch, or $6 per board. They use Eagle as their CAD software. Their boards look good (I think silk screening is included, but I'm not sure).
Futurlec PCB Service: Futurlec has a PCB service. There is a $25 setup cost per run, but for a 2"x3" board, it's $3.1 in quantities of 100 and $3.8 for 1. They can also do solder masks and silk screening. With silk screening and a solder mask, it's $3.71 for 100 boards.
Fritzing Fab: They charge by cm^2, which means that I have about 38 cm^2 for a 2"x3" board. Even in quantities of over 200 cm^2 (about 5 boards), it's $20 per board. They are a lot more expensive than the other suppliers. They use Fritzing though. (for over 10, it's around $17).
PCB advice: Here's a blog about making PCB's for pick and place machines to pick and place onto. It also has other PCB information that you could use.
Outsourcing PCB manufacturing:
Ch00ftech has an article about outsourcing the manufacturing process for PCBs entirely. It covers using Myro, a Chinese manufacturing company, to make the PCBs and assemble them. It covers a lot of good information if you don't want to hand solder a whole bunch of boards.
Seedstudio also has a service where they manufacture the PCB's and put all of the components on them. They have you purchase the materials for them (and have them sent there), but it's another option instead of just looking at Myro.