Cost per defect is the inverse construct to inspection efficiency, i.e., the effort to find a defect (e.g., 20 person minutes per defect).

Biffl S., Halling M.: "A Knowledge Management Framework to Support Software Inspection Planning", In: Aurum A., Jeffery R., Wohlin C., Handzic M.: Managing Software Engineering Knowledge, Part 3, pp 231-249, Springer Berlin Heidelberg, 2003. [Springerlink]