I was researching how to add simple sensors to Max when I ran across this.
This article will describe how to build and program a custom arcade button MIDI controller, while trying to keep the price below $100. It is aimed at electronics and programming novices.
In the comments of the project the original author recommends a different micro controller.
The "Teensy" board can act as a true USB-MIDI device, so there's no need for the extra software layer the Arduino requires to convert serial messages to MIDI. That's the biggest advantage the Teensy board has. The Teensy's other advantage is that it has more I/O than the Arduino. If I were to remake this controller, I'd definitely use a Teensy or similar ATMega32U4 based dev board, simply for the usb midi class compliance.
This brings the cost down even further. I haven't soldered anything since I was 15 years old. Maybe it's about time.