actually they are 400mm f/2.8, 500mm f/4, or 600mm f/4. if you ever see a lens the size of a car, that is the Nikon 800mm f/4 lens for 120,000$, the other three lens go for, respectively, 8,000$, 9,000$, and 9,000–10,000$. I am speaking for Nikon only, I know Canon is cheaper.
I use both Zoom and Fixed only because I fully utilize the features of my D300. I can tell the firmwear (the camer’s brain basiaclly) to auto-focus diffrently for every one of my lens. It was about two days of sold work to get, but the pay off is more then worth it.
You also have to consider that Fixed lens don’t brake down as fast, seeing as they usually have fewer groups and fewer moving parts, in turn. The glass is usually better too, due to the fact it can be a little bit heavier here and there. Also one other factor is most Fixed lens are sealed, whether they advertise that or not. With a Zoom lens it takes more to seal it from the elements and thus, more money.