The story I heard was that Fedex mandated Boeing develop a 777-200 P/F conversion plan if they were to purchase new 777 freighters. Unfortunately, they didn't put any limits on the price of the conversion. Rumor has it it that the 777-200 P/F conversion may cost $100 million per aircraft. There's something about a major renovation to the weight bearing capacity of the main cargo floor that requires an involved tear-down and rebuilding.