[Silhouette]

There was a popular 90s report that concluded a 10× cost for each step from requirements to design to testing to production; I suspect that’s the one.

That rings a bell. Possibly it was something cited in the original Code Complete? Could also have been cited by one of the consultants pushing (at the time) relatively new and unusual testing and review practices as evidence for why their advice was worthwhile. But you're right, I think the exponential cost increase argument was based on a very waterfall-y development process.