I was just thinking about how to do a synth like the RND from Cyma Forma ( thread ), using the $29 AMYBoard ( post ) ( link ).
Fun thought experiment. The built in Micropython on the AMYBoard should make this fairly easy to do.
Doing your own would allow for your own custom choices of synth engines, sequence generators, effects, control interfaces, etc.
The RND is very inexpensive, but this option would cost even less.
Plus you could add a back button !
How hard would it be to add a MIDI looper to act like the Svensson from Auxy ? ( thread )