An amazing project from Travis Brown. He was able to fit a raspberry pi and a 2.5 inch TFT into a gameboy pocket case. Total cost was around 140$ and this includes except raspberry pi and tft, a Teensy microcontroller, a Li-Ion 2600mAh battery, an audio amplifier, regulator, chargin board etc.

You can find all the details here: http://www.xodustech.com/projects/raspberry-pi-gameboy-pocket