DIY Arduino
2019
Build your own arduino for about $5 in parts - Julian Ilett -
https://www.youtube.com/watch?v=sNIMCdVOHOM
BOM
- USB to serial - [1]
DIY Arduino Mega
- Requires surface mount, but is empowering when done correctly. This one didn't work because you can't program it? - https://www.instructables.com/id/DIY-Arduino-Mega-2560/
- Another - can do it - essentially breakout of the microprocessor chip. Is it programmed with the simple plug? [2]
2011
Arduino can be used a very popular open hardware component and can be successfully used in lots of open source projects:
Although you can always make your own arduino using a breadboard using the following instructions: 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.