I'm no camera expert, or even a novice for that matter, but, I've heard that lenses for Nikons are more expensive than those for Canons due to the location of the drive motor (or something like that). Something about the drive motor being located in the lens for Nikons, and in the body for Canons. i.e. Everytime you buy a lens for a Nikon you are paying for a new drive motor as well.
This is just something I've heard, and have no evidence one way or the other. Take it with a grain of salt.