You can make Johan Von Konow's new modular synth for about $6!
"I have designed a keyboard, drum pad, chord keyboard, arpeggiator and a step sequencer. One special feature is that the units have RGB LEDs for each key, enabling playback visualization (so each device is both MIDI out and in). This is helpful for music training and editing, but it also looks great. They can be used as input devices to any computer with a DAW (Digital Audio Workstation) like Ableton, Logic, Cubase, Garageband etc. It will run on Windows, Mac or Linux (including Raspberry Pi). They can even be connected to your mobile phone (Android or iOS), providing a tactile super portable music development platform."by Johan Von Konow
The devices are easy to build and anyone with a 3D printer and basic soldering skills should be able to replicate them. They are designed to use few components that are easily accessible and affordable (one keyboard cost around $6).
Everything is open source and built with the easy to use Arduino framework.