We develop a holistic cost model that operators can use to help evaluate the costs of various routing and peering decisions. Using real traffic data from a large carrier network, we show how network operators can use this cost model to significantly reduce the cost of carrying traffic in their networks. We find that adjusting the routing for a small fraction of total flows (and total traffic volume) significantly reduces cost in many cases. We also show how operators can use the cost model both to evaluate potential peering arrangements and for other network operations problems.