Critter and Guitari‘s Keypad Cigar Box Synthesizer is described as the best synth for $100!
Here’s a preview of what this boxed beast sound like:
This is a synthesizer with keypad pitch control. The 16 keys play chromatically, so you get an octave plus 3 notes. It produces sound using a simple frequency modulation oscillator pair. The four knobs control volume, tuning (shifts the base pitch of the keypad up and down), harmonicity, and modulation-depth-waveform. The modulation-depth-waveform is a combo-control that varies the modulation depth and changes the waveform of the oscillators from sine to triangle.
It is a nice and loud instrument, powered with a 9 volt battery. A 1/4″ switched output jack bypasses the speaker and allows the synth to be plugged into an external amplifier / effects.