We present a case study that aims at quantitative assessment of the impact of requirements changes, and quantitative estimation of costs of the development activities that must be carried out to accomplish those changes. Our approach is based on enhanced traceability and an integrated view of the process and product models. The elements in the process and product models are quantitatively characterised through proper measurement, thus achieving a sound basis for different kinds of sophisticated analysis concerning the impact of requirements changes and their costs. We present the results of the application of modelling and measurement to an industrial project dealing with real-time software development. The ability to predict the impact of changes in requirements and the cost of related activities is shown.