If you are starting with nothing (no type of camera, no type of monitor), a budget of less than $100 really is unrealistic. However, there are a couple of options worth considering.
A cellphone was already mentioned. However, rather than crowding a whole class around a small cellphone, you can probably project the cellphone screen to a larger television or monitor either by cable or screen mirroring. How you accomplish this will vary greatly depending on the cellphone used and the monitor/television used.
Alternately, if you already have access to a newer camera, particularly a mirrorless camera with a mini-HDMI out port, you can connect it to a monitor with an HDMI jack. This is assuming you have access to a monitor with an HDMI port.
When my grandchildren visit and we plan to do some microscopy, I have found that swapping places to see through the eyepiece is a bit of a waste of time. This gets far more complicated if the microscope is a binocular/trinocular microscope like mine. Adjusting the pupillary distance of the eyepieces slows us down, refocusing slows us down, swapping seats slows us down, and not being able to easily point out features is not great for a whole group. I put a T-ring with a 23.4mm nosepiece on my Sony a6000 and then plug a mini-HDMI to HDMI cable from the camera to a monitor. The T-ring, appropriate sized nosepiece and cable would cost less than $100 but you would need a camera and a monitor/television already.
Worth noting, however, is that this type of prime imaging isn't really ideal for attaining the best image quality. Well, at least not in my experience with my particular gear. I found that prime imaging this way was rather soft compared to my view through the eyepiece. Prime imaging would be good enough for pointing out larger cellular features to a larger group but it is not the way to focus on fine details or get the best image quality possible.
I found that the eyepiece projection imaging method worked best for microscopy. This, however, probably would not work well with a smaller monocular microscopy (it sounds like this is what you have available to you) due to the weight of a camera mounted on the eyepiece. This off-balance weight would likely topple a smaller microscope.
However, you could do afocal imaging with a cellphone and then somehow project the image to a nearby monitor or television. Once you mount the cellphone on the eyepiece, you simply zoom in to eliminate the severe vignetting. You'd need a cellphone bracket/clamp that fits your particular eyepieces and a way to project your cellphone image to a monitor/television.
Let us know which solution worked for you.
Edited by PatrickVt, 12 May 2022 - 08:49 AM.