This is a question I often asked, well about all iPhone models since my iPhone 3. Below is a link to a post by Wendy Tietz which includes good estimates. It is set out like a student exercise too, so give it a go if you have the time.  Seems Apple make quite a margin, and it seem the mobile operators subsidise quite a bit too.

What is the gross margin on the Apple iPhone 5S and 5C?.