This is the 2010 update for those of you in the market for a student microscope. Some companies have dropped their prices making the decision-making process more difficult.
For an explanation of the parts I mention, you can see my original post or this guide.
The microscope with the most features and the best price is Bargain Microscopes' Student 1000x. Besides having the additional 1000x oil immersion lens, it has professional-grade halogen lighting, coaxial focus nobs, iris diaphragm, and a mechanical stage. It costs around $245 with shipping, far less than other 1000x scopes. (Oil immersion is really great for looking at blood cells and other objects of similar size.)
You don't need an oil immersion lens or halogen lighting for high school biology, though. The Sonlight Ultra Microscope has the same features only with fluorescent lighting without the 1000x lens for $230 with shipping.
If price is the biggest factor you can get less expensive microscopes if you give up some combination of fluorescent lighting, mechanical stage, or iris diaphragm but you won't be as happy (in my opinion, anyway.)
GreatScope's SAF3 has fluorescent lighting, iris diaphragm, and a clip stage for $211 with shipping.
Home Training Tools' Home Microscope has fluorescent lighting but a disc diaphragm and a clip stage for about $187 with shipping. A mechanical stage costs $25
Bargain Microscopes' Student 400x has tungsten lighting, disc diaphragm, and a clip stage for $175 with shipping.
I could not find a scope with tungsten lighting and an iris diaphragm.
For something a little more eclectic you can look at Bargain Microscopes' Rechargeable 400x and camera. The microscope has LED lighting (takes less power and is a step up from fluorescent), a disc diaphragm, and a clip stage; it is also rechargeable so you don't have to be near a plug. It has a camera that you can insert into your eyepiece and plug into your computer with a USB port. It captures video and still images. The package is $210 with shipping.
That's the round-up for this year. I hope you find something you enjoy!