There is a general agreement among software engineering practitioners that software inspections are an important technique to achieve high software quality at a reasonable cost. However, there are many ways to perform such inspections and many factors that affect their cost-effectiveness. It is therefore important to be able to estimate this cost-effectiveness in order to monitor it, improve it, and convince developers and management that the technology and related investments are worthwhile. This work proposes a rigorous but practical way to do so. In particular, a meaningful model to measure cost-effectiveness is proposed and a method to determine the cost-effectiveness by combining project data and expert opinion is described. To demonstrate the feasibility of the proposed approach, the results of a large-scale industrial case study are presented and an initial validation is performed.