These 50×70 cm protoboards seem to fit perfectly in the $0.24 case. My idea is to mount them to the cover, but they can almost fit anyhow you want them.
Let’s say you have 2 boards for a particular project. Total cost of box + boards will be $1.50, add a MCU (ATMEGA328) and it’s ~$5.50. Lets add regulation and bring it to a total of $7 (estimate), That gets you 2 boards + processor + voltage regulation + well built case to hold it all. You can make 4 of these for the price of 1 Arduino Uno. While it may not be as pretty, it is very cheap and let’s you explore new and strange ideas, because all you’d be risking is $7, not a $30 board + $15 breakout board.
First board I’ll make will be a XBee adapter, to bring XBee to any breadboard in need. I’ll also put a ISP programming pins on there, since my current ISP programming setup is kinda strange.