Query processing in P2P environment is gaining more and more attention in database research community, and most of the P2P-based query processing systems only consider the processing strategies. However, estimating the cost of query processing is crucial to clarify which factors will affect the efficiency of query processing, and how to optimize the query processing. In this paper, a model is proposed to estimate the cost for query processing in P2P systems. Methods in the model do not depend on the global information in the network, and they only need a small quantity of data transferring to complete the estimation. Experimems are presented to evaluate the efficiency and the effectiveness of the estimation methods.