Cheap is relative, I guess. You can find a used stereo microscope on eBay for well under $100, maybe under $50. Building one out of a pair of binoculars is a classic but relatively advanced DIY project. I’m pretty sure there’s a detailed write up of one in either the old Edmund's optics books or Scientific American’s amateur scientist collections. Something like this:
https://web.archive.org/web/20190123040421/http://www.funsci...
If you’re thinking of something like a metallurgical microscope, those are more involved and expensive, but again, eBay.
If you’re thinking of something like a metallurgical microscope, those are more involved and expensive, but again, eBay.