I'm so confused! Can someone please explain the difference between lenses and their prices?

It all comes down to quality. What would be best to help you understand is if you shot over to a reputable camera store and had these three lenses in your hands. The 70-200 2.8 is a favorite of mine (great on the D700, I've got both). What you'll notice is that the 70-200 2.8 has a sonic drive motor in it that focuses almost faster than you can detect, while the other two will focus slower and be considerably louder. If you've got the money, I'd go for the 70-200 f2.8. The reason being there will be no limitations, creatively or with your equipment. This combination of camera and lens will almost see in the dark if electronic noise doesn't bother you. Good luck!