Service Overlay Network (SON) was proposed to alleviate the difficulties encountered in providing end-to-end Quality of Service (QoS) guarantees. SON is able to provide QoS guarantees by purchasing bandwidth from individual network domains and building a logical end-to-end data delivery infrastructure on top of the existing Internet. We focus on SON topology design problems under a generalized cost model. Earlier research in this topic considered two distinct cost models fixed (leased) cost model and variable (usage-based) cost model. However in most applications, the costs of both nodes and links have a fixed component as well as a variable component that often depends on usage. Our generalized cost model takes this fact into account and our topology design algorithm uses this cost model to find the optimal topology. Since the SON topology design problem is NP-complete, we provide approximation algorithm with guaranteed performance bound. We validate the effectiveness of our algorithm through extensive simulation.