The publish/subscribe communication system has been a popular communication model in many areas. Especially, it is well suited for a distributed real-time system in many ways. However, the research of cost model and analysis of publish/subscribe system in a distributed real-time system have not been suggested yet. In this paper, we present our cost model for publish/subscribe system in a real-time domain, analyze its performance, and compare it with other communication models such as request/reply and polling models. Our empirical result on mobile embedded device shows accordance with cost analysis, which verifies correctness and usefulness of our cost model.