How come Boeing invested \$32 billion on the 787 program and Airbus only \$15 billion on the A350?
I don't think the breakdown is public, but there are two main areas that are obvious once you dig into the details.

Part 1 relates to the 787-8 and -9/-10 commonality. Surprisingly, the stretched derivatives only share 50% of the -8 parts. As Wikipedia puts it:


  The 787-8 and 787-9 have 50% commonality: the wing, fuselage and systems of the 787-8 had required radical revision to achieve the payload-range goals of the 787-9.


So you could say Boeing developed 1½ planes, while Airbus developed 1. I tried to find details on the differences before, but it was a futile attempt. All the official statements were as vague as they come. (Boeing will implement at least some of the new -9/-10 developments into the future -8's.)

Part 2, Boeing went with an all-electric design,* introducing many new technologies in one go. The A350 still uses pneumatic engine start and bleed air for pressurization. They were improved on the A350, but still they were existing technologies.



* Lithium-ion batteries are not really part of the differences as they were introduced quietly on the A350.