Littleprince
I've no idea how much it would cost but I suspect an awful lot, and I accept that commonality is not as important as it once was. But customer preference is. From personal knowledge I can tell you that the B777-300ER would have picked up a lot more orders more quickly if it had also been offered with Trents as well as / instead of GE90s and Boeing must be very grateful that the A346/5 are no real competition at all!
Presumably, Boeing must have decided that a second engine type on the B744Adv would not add sufficient incremental orders to justify it. Of course, these things have a habit of becoming self-fulfilling prophesies and I suspect they'll be "proved" right i.e. restriction to a single engine type will limit total sales and we'll never know what would have happened if they'd offered the same engine choices as on the 787.