DIY Arduino

Arduino can be used a very popular open hardware component and can be succefully used in lot's of open source projects:

Although you can always make your own arduino using a breadboard using the following instuctions: http://itp.nyu.edu/physcomp/Tutorials/ArduinoBreadboard

A similar project could be making you own arduino using a perfboard. http://hackduino.org/mapblog/?page_id=46

Another take would be to create YOUR OWN Arduino PCB using following instructions on this project http://www.theparsley.com/arduino/diy/

In terms of cost most DIY guides featured here will cost you more money (in you add up the labor cost) than a prefabricated arduino board. But for larger quantities the guides could be cost effective. Although using a perfboard or ever a breadboard for could be useful for prototyping using a PCB (DIY of prefabricated) could be more a more practical solution for finished products.