Fog application design is complex as it comprises not only the application architecture, but also the runtime infrastructure, and the deployment mapping from application modules to infrastructure machines. For each of these aspects, there is a variety of design options that all affect quality of service and cost of the resulting application. In this paper, we propose an approach for evaluating fog-based IoT applications already during an early design phase. Our approach relies on modeling and simulation to provide estimates for quality of service and cost so that developers can interactively choose a good application design.