Engineering of modern complex real-time systems is driven by a host of design objectives, often differing not only in nature, importance, and form, but also in dimensional units and range, and themselves interacting in complex ways. These objectives influence design decisions throughout the entire system life cycle. We consider the problem of formulating the cost of a design decision driven by multiple objectives. We start by discussing issues and relationships among non-functional objectives, their decomposition, and attribute cost functions used to quantify them. We then describe a hierarchical model, along with an equational form, for system cost function synthesis. Finally, examples are given to show how the model applies to actual multi-objective problems in the design of complex real-time systems.