It's partly due to manufacturing differences (small part), mostly due to market position. Intel can charge more because its brand is accepted. Most major corporations will only buy Intel and the corporate market is where the bulk of the sales are made.
AMD's market has traditionally been the smaller manufacturers and computer stores and, because their brand name is not linked as strongly to quality and stability as Intel, they have to charge less. The volume buyers of technology do not care about performance, they only care about stability and, for the purchasing agent, covering his anatomy.
If AMD, with T-Bird and Duron, sees their brand name rise in quality, and, more importantly, major PC manufacturers order lots of them, then you will see their price go up.
As Yoshi said, corporations have no feelings. Like sharks, they only exist to eat and sh!t.