According to a Digg.com article it costs Apple $245.84 for a 4 gig iPhone and $280.83 for the 8 gig version. The information supposedly comes from iSuppli Corp but the image for the article is missing. I doubt that they took into account the R&D that Apple put in as well as the manpower to manufacture. This may just be a raw materials cost.
Read more here:
News.com: Price cuts on Apple iPhone likely, analysis finds
iPhone Costs Only $245.83 to Build
