Link cost is very important in media distribution trees. A good cost function can provide information for media routing algorithms to nd the best way to distribute the media on overlay networks. In this research, a bandwidth-type cost function is proposed. The proposed cost function can calculate links' costs based on both network resources and application's requirements. It can help Application Layer Multicast (ALM) routing algorithms to avoid congestion before building media distributing trees. The derivation process has also been explained in details so that it can be further applied in other conditions to build other cost functions suitable for different requirements. The newly proposed cost function will then be applied in a popular Application Layer Multicast (NICE) to replace its old cost function. Possible modifications to NICE's algorithm will be discussed. Intensive simulation scenarios have also been carried out to validate the advanced performance of the newly proposed cost function in comparison with conventional functions. The simulation scenario has also been developed to show the adaptation of the cost function under real conditions when some peers use the WiMAX access network to join the multicast group.