The publish/subscribe communication system have been popular communication model in many area. Especially, it is well suited for distributed real-time system in many ways. However, the research of cost model and analysis for publish/subscribe system in distributed real-time system has not been announced yet. In this paper, we present our cost model for publish/subscribe system in real-time domain, analyze its performance, and compare to 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.