The main reason that the iPhone is twice as expensive as the iPod Touch is nothing to do with what is inside either device. The price difference is mainly down to how they are sold.
Most iPhone are sold with an associated mobile contract. For some reason people are willing to pay more for a device if the payments for it are spread over a couple of years built into their phone contract. So a significant chunk of the extra cost of the iPhone comes down to the fact that that is simply the cost that people are willing to pay for it.
The iPod Touch in comparison is not "subsidised" in the way that most iPhones that are sold are (it is a very odd definition of subsidised that the mobile carriers have).
That isn't to say that the extra components, software and regulatory issues around the iPhone don't add some cost.