The number of (true) defects detected per unit of effort, e.g., person hour (3 defects per person hour). 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]