I hope it is OK to mention a brand-name product on this site. If that's a no-no, I apologize and won't do it again.
Last year, I made an impulse purchase in a computer store, and it was the best $80 I ever spent. It's called the Digital Blue computer microscope, and while I'm not a biology teacher, I would guess that this would be the best classroom tool you could possibly find for that little money.
It hooks up to your computer with a USB connection. You put your specimen under the microscope and sit back and view the image on your computer screen. Magnifications are 10X, 60X, and 200X. The resolution is surprisingly good. You can use it for viewing live, but you can also take still pictures and time-lapse video. You can also use it as a Web cam. It is much cheaper and easier to use than an optical scope, and a whole classroom can watch it at once.
I've taken great time-lapse clips of monarch caterpillars hatching and consuming their egg-shells. You could also observe molting, etc., and look at look at butterfly wing scales, veins, mouth and leg parts, etc. You could also use this with a computer projector, of course.
I am NOT associated in any way with the company that makes these, and I generally don't indulge in free advertising. But this has become my favorite toy, and I think it's worth sharing.