The Software Product Line approach undertakes the development of complete portfolios of software products as a single, coherent development task. Although there are well-documented examples of cost reduction, shorter development times, and quality improvement achieved by introducing the product line paradigm in industry, the approach is not always the best economic choice for building a family of related systems. To support decision makers, a number of economic models have been proposed. Apparently, existing proposals are rather heterogeneous in terms of their main characteristics and goals. Nevertheless, this paper shows how most models may be defined using a small common lexicon. Translating models to such common lexicon, we compare them in detail, identifying their strengths and weaknesses. As a result, the paper proposes an integrated model that improves the cost estimation accuracy of existing models.