The most expensive part of the HTC ThunderBolt is its Qualcomm MDM9600 4G LTE baseband chip. That piece of detective work leads iSuppli to compute that an LTE version of the Apple iPhone would cost 23% more to build ($211.10) than the current 3G model, assuming that Apple uses the same first-gen LTE parts that are in the T-Bolt. Wayne Lam, a senior analyst at IHS said that Apple would need to increase the size of the printed circuit board inside the iPhone in order to support the LTE baseband processor and the accompanying chipset. This dovetails perfectly with comments made by Apple COO Tim Cook who stated that Apple did not make the current version of its handset LTE enabled because it would have required a whole redesign of the slim model, which would have added some heft to it.
source: IHS iSuppli via PCMag