Video conferencing service requires a multicast tree to distribute its multimedia contents to all participants. Link cost is very important in building such media distribution trees. 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. Since participants may join the conference using either a high speed wireless network such as WiMAX or fixed (heterogeneous) networks, we construct a scalable video conferencing service on an overlay network of a simulated Internet topology and a real WiMAX network and apply the newly proposed cost function for building the multimedia distribution tree for the service. Some participants join the conference from the WiMAX network and others from the Internet. An intensive evaluation platform has been built to evaluate the performance of the newly proposed cost function. The collected real measurement data have validated the advanced performance of the new cost function in the rapidly changing heterogeneous network environment.