The 18-5mm lens (and others like it) are designed for digital SLR camera that have a sensor which is smaller than a frame of 35mm film.
When you think of 18mm as 'really wide' that is true...on 35mm film cameras, which have been the standard for many years. Today, most DLSR cameras have sensors that are slightly smaller, which results in a crop of the field of view...which gives us the 'crop factor'. On most Canon DSLR cameras, the crop factor is 1.6. So when you use a 50mm lens, the FOV is 'cropped'...and you get the same FOV as an 80mm lens would have on a film camera. So 18mm on these cameras, really only has the FOV that you would expect from 28mm on a film camera...which isn't really very wide.
If you want a really wide angle, you need something like the Canon 10-22mm lens.
You can still use 'fish eye' lenses...but the edges of the image through the lens would be 'cropped'...so you loose a lot of the effect. I beleive that Sigma has a 10mm fish-eye, which would work on these 'crop cameras'.
maybe i worded that wrong, so 18mm is considered "wide angle" why is a 18-55mm lens so cheap, yet a fixed lets say 22mm lens so expensive?