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Transit traffic exchanged with transit providers costs more for network providers compared to intra-domain traffic or traffic exchanged over peering links. We have measured and analyzed the peer distribution in BitTorrent, which is one of peer-assisted content delivery networks (CDNs). From the peer distribution, we show the potential of the high-cost transit traffic reduction. We then propose a peer selection preference which takes into account the economical relationships among Autonomous Systems (ASes) in peer-assisted CDNs to reduce the high-cost transit traffic. Since most commercial Internet service providers cannot disclose the relationships due to their commercial contract, we employ degree-based heuristics for inferring the relationships; degree can be approximated from publicly available BGP routing tables. We show that the peer selection method utilizing the proposed preference can reduce interdomain transit traffic exchanged with provider ASes by trace-driven computer simulation. The significance of this paper are 1) we show the potential of the transit traffic reduction from peer distribution analysis, and 2) the peer selection method with the proposed preference appropriately reduces the high-cost transit traffic with degree-based AS relationships inference heuristics even though there is no public AS relationships information.