A microscope for the interested amateur should not be too expensive, but should be capable of being upgraded should the interest (or wallet) grow. This means it must have a sub-stage condenser, since a microscope without one cannot be upgraded to higher power objectives or powerful contrast enhancing devices.
To save money, the microscope need only have three objectives in the turret (4x, 10x, and 40x). It can be upgraded by adding a higher power objective later, at the expense of one of the first three. If you need one of the lower magnifications, you can always screw it back in to replace the new high power objective, at the cost of some convenience.
You can also give up binocular eyepieces to save money. On many microscopes, you can swap a monocular eyepiece head for a binocular one later, when the budget is less tight, or the interest level increases. Or you can add a video camera and watch the show on a computer monitor or large screen television.
All that said, a good microscope with binocular eyepieces and four achromatic objectives (4x, 10x, 40x, and 100x oil immersion) is not out of reach financially, costing about what a nice high end consumer digital single lens reflex camera would cost. Or a mid-range lens for that camera. That 100x oil immersion lens won't get you its full value unless the sub-stage condenser can also do oil immersion, so that is something to check for.