Packsonflight, Fropilot;
It should be very creative to come up with such development costs that would bring a cost of derivative close to cost of a new project. Ratio is much closer to 1:10.
Even if you want to be conservative you go with 20% which is really high. Assuming that some of the cost will be offset to 787 work already done it is not surprising that Boeing is trying to spread the new stuff to its current programs.
747 is the logical example since it has the good capacity and fact that they sold cargo prior to passenger does not mean it is carried by the cargo version.
it is simply question of who reacted first, in this case. Cargo a/c are seeing boom in the last few years so there is a lot more demand form them then for pax a/c.
Cheers,