Bright field microscopes, which are used for routine laboratory work, cost about $500. If you want to have phase contrast optics, then expect to pay about $2000 plus.
The cost of a compound microscope is largely determined by the objectives attached, the extensibility of the microscope and overall stability. It is well possible that individual components of the microscope cost more than the rest of the microscope together. So it does pay off to be clear about ones own needs and observation interests.
Stereo microscopes might be a bit cheaper, but here too there is no upper limit. Anything costing much less than $150 is most likely not of sufficient quality, unless it is a well maintained used microscope. Avoid toy microscopes whipch are mostly made of plastic, and which often do not have DIN optics. They will have a low resolution and be difficult to focus.