I recently built the JYE Tech DSO-150 Shell oscilloscope kit (#15001K) that I got from Banggood for $24 plus a power supply. These are single-channel, low-frequency scopes, but would be useful for things one might do with an Arduino or similar microcontroller, such as stepper motors and servos, plus of course general audio stuff. The bandwidth spec is 200 kHz, but I suspect the display would be pretty distorted at that frequency.
There are a number of videos posted on Youtube on the build process and operation of these scopes, and I posted one on Vimeo showing how it could be used to monitor the PWM output of a TI controller.
Dave Jones of the EEVblog says these kit scopes are garbage, but I think that's too harsh. For low frequency stuff they work fine. But of course they do suffer in comparison to a new $400 digital scope, or even a used analog scope, but most people this side of an EE degree just don't need such a fancy scope.
Well anyway, for $24, it's a fun project, and it can be useful for troubleshooting. The kit instructions are pretty good.