Note this applies to the manufacturing,not the design.

It is actually a wonderful analogy with software:

* to a first order, cost of the materials in an airplane are proportional to the weight * at design time, the weight of the aircraft is minimized. thus, political and management forces cannot change it.

just like software, each subsection that is bolted on has different weights; but if you average it out it is a good measure of progress. and cost.