Here's a low cost teaching microscope designed to be built and used by students to learn microscopy. Depending on your choice of lenses, this can cost between $0.40 and $3.50 per microscope.
Instructions Watch the video for assembly instructions. Please note that this version is slightly different from the one in the video. The extra slide stand has been removed and integrated it into the comb. The lock ring has been removed (everyone had trouble with it) and replaced with a clip that snaps snugly over the end of the focus axle. Use a 3mm opening for the objective. You can hold the lens in place with a little dab of super glue. Try to keep the glue on the edge so that it does not affect image quality.