Real time video multicast streaming services on overlay networks need to build a multicast tree for distributing the video contents to all clients. Link cost is very important in building this multicast tree. A good cost function can provide information for media routing algorithms to find the best way to distribute the media on overlay networks. In this research work, a multi-variable cost function is proposed. This cost function can calculate links' costs based on both network resources and application's requirements. The new cost function is then applied to build an overlay network for scalable video multicast streaming services of up to more than one thousand peers. An evaluation platform is proposed to validate the advanced performance of the newly proposed cost function in overlay network. The results have shown a better adaptability of the new cost function in the scalable video multicast streaming service on overlay networks.