salt 200x (click to enlarge)

Celestron will supposedly announce a sub-$300 dollar microscope with a built-in screen and a 2 megapixel camera. That's pretty cool, but it that's out of your price range, you can pick up an used IntelPlay QX3 microscope on eBay for $35. The specs aren't quite a sweet (200x, 1 megapixel), but it does hook to a USB port.

I've had one for many years. I found an OS X software package called macam that works with it pretty well. My son and I took some pictures with it a while back.

If you're looking for a little microscope fun and don't want to blow $300, this set up will suit you just fine.