Cost function is an essential part in Application Layer Multicast (ALM) routing algorithms. It is from a cost function that we can calculate links' costs and then build the data delivery tree for multicasting. Unfortunately, cost function remains an almost untouched research area in ALM routing. In this research, we propose a new multi-variable cost function considering various end-to-end QoS parameters simultaneously. The mathematical derivation process is also described in details so that one can apply it to obtain other multi-variable cost functions according to their specific requirements. The newly proposed multi-variable cost function can avoid congestion before it happens, preventing the data delivery tree from being frequently or unnecessarily changed while still be adaptable to the dynamic requirements of different applications. With theoretical analysis, we have proved that the new cost function can provide better performance for ALM routing algorithms compared to conventional cost functions.