1. Boehm, B. W. (1981). Software Engineering Economics. Prentice-Hall. In Chapter 4, "The Cost of Software," Boehm presents seminal data showing the relative cost to fix errors increases dramatically from the requirements phase through the maintenance phase. A defect found in the design phase is orders of magnitude cheaper to fix than one found during system testing.
2. Fagan, M. E. (1976). Design and code inspections to reduce errors in program development. IBM systems journal, 15(3), 182-211. https://doi.org/10.1147/sj.153.0182 This foundational paper on software inspections provides data showing that formal inspections can find 60-90% of defects before the first test is ever run, demonstrating their high cost-effectiveness (see Section: "Inspection results," pp. 196-200).
3. Carnegie Mellon University, School of Computer Science. (2016). 15-214 Principles of Software Construction: Objects, Design, and Concurrency, Lecture 1: Intro & Quality. Slide 22, "Relative Cost to Fix a Defect," visually represents the exponential cost increase, illustrating that a defect found post-product-release can be 100-1000 times more expensive to fix than one found during the design phase. This supports the economic argument for early detection via inspections.