[agilepixels]

Weather sealing and better plastic of L-lens is worth usually ~$100-$150 (according to several references) and majority of the price difference is in glass and corresponding assembly and verification process at the factory.

Most of the high-end L-glass have mostly manually controlled assembly and testing process. So, here's the price...

Regards,
Nikita Ivanov.
http://www.flickr.com/photos/agilepixels/