In recent years, with the advent of the Internet of Things (IoT), fog computing is introduced as a powerful complement to the cloud to handle the IoT's data and communications needs. The interplay and cooperation between the edge (fog) and the core (cloud) has recently received considerable attention. In this paper, we consider task scheduling in a cloud-fog computing system, where a fog provider can exploit the collaboration between its own fog nodes and the rented cloud nodes for efficiently executing users' large-scale offloading applications. We first formulate the task scheduling problem in such cloud-fog environment and then propose a heuristic-based algorithm, whose major objective is achieving the balance between the makespan and the monetary cost of cloud resources. The numerical results show that our proposed algorithm achieves better tradeoff value than other existing algorithms.