Software cost estimation and overruns continue to plague the software engineering community, especially in the area of safety-critical systems. We provide some preliminary models to predict the cost of adding fault detection, fault-tolerance, or fault isolation techniques to a software system or subsystem if the cost of originally developing the system or subsystem is known. Since cost is a major driver in the decision to develop new safety-critical systems, such models will be useful to requirements engineers, systems engineers, decision makers, and those intending to reuse systems and components in safety-critical environments where fault tolerance is critical.