Nowadays P2P is the premier data distribution technology which tops Internet traffic flow chart. It becomes more and more important to optimize the data flow of P2P traffic. However, there is no general standard for measuring which data distribution scheme is better. A network expense model is proposed for this purpose. By observing the data flows which are produced by P2P, we can calculate the network expense of any data distribution scheme. Now there is a unified standard for evaluating the influence of different data distribution schemes to the network. According to this model, the network expense of a few typical P2P data distributions scheme over the Internet is calculated. Judging from the results, a topology-aware P2P can generate more balanced flows to save the bandwidth of backbone network. Network expense model provide a fundamental tool for optimizing the P2P data distribution.