Melody example:

Bass example:

Playing bass by singing falsetto:

It consists of:

• Raspberry Pi 3 computer ($35)
• Case ($7)
• USB sound card ($8)
• Keypad ($10)
• 3.5mm to 1/4" cable ($4)
• XLR to 3.5mm cable ($7)

It also needs a micro SD card, power (2A, USB), and a microphone. I'm using ones I already had, but it doesn't need anything fancy. If you wanted to make a special-purpose device, I think you could get the materials down below $20; there is nothing especially expensive here, and I'm not using most of the features of the Pi.

The code is open source: github.