The DSO138 oscilloscope is such a device - you can get one for $25 on eBay!
This is a 200kHz-sample-rate oscilloscope that is normally sold as a solder-it-yourself kit for students and hobbyists to assemble. However, if you don't feel like soldering such a thing together, you can just buy a pre-assembled unit. This is what I did, as I'm way more interested in a working scope than yet another soldering project.
Output is on a built in 320x200 LCD, and the system supports from 10mV to 5V per division, and a timescale from 10us to 500s per division. It also has some built in analysis and a signal buffer that one can use to freeze a waveform and look around.
Is it fancy? Certainly not. Is it $25 shipped? Yup! I'm willing to forgive a good bit for that price.
I've spent a bit of time with mine. There are plenty of reviews and thoughts in video form, but my review is in text form, because I don't like using video for things like this - sorry, everyone who wants me to do video. I've also assembled the standard laser-cut acrylic case, and I made my unit USB powered instead of 9V powered, because I like things that run on USB battery packs. I have enough USB battery packs laying around (Ingress player and all) that I can run USB devices anywhere, for absurdly long periods of time.
Interested? Keep reading!