"
according to The Wall Street Journal, the U.S. Air Force has set aside $1.65 billion between 2015 and 2019 for two replacement jets."
Base price for a 747-8 is in the $350-$375mm range - but they have to design, fit and certify a lot of extra kit . Air-to-air refueling is probably the most obvious but all the electronics probably have to be hardened and all the comms gear will cost an arm and a leg.
the current VC-25 is quoted at a unit cost of $325 million cp the commercial cost of around $ 80mm per commercial aircraft so the extra cost look as if it is in proportion