Due to the increasing bandwidth demand of mobile users and their devices with energy hungry wireless networking modules, attention of research efforts has been recently shifting to find answers to the paradox of achieving more spectrum for less energy consumption. In this paper, cognitive radios have been employed to obtain more spectrum by utilizing unused licensed spectrum in an opportunistic manner. Defining the opportunity cost as the consumed energy per achieved unit of free spectrum, we propose a cooperative sensing scheduling framework to optimize the cost with the consideration of the sensing, reporting and channel switching costs in terms of energy expenditure subject to a licensed user protection threshold. In the proposed scheme, all primary channels are scheduled to be cooperatively sensed within a cycle which consists of rounds. In every round, secondary/unlicensed users (SUs) are first assigned to cooperatively sense the scheduled primary/licensed user (PU) channels. Consequently, SUs report their local sensing results to a fusion center for a global decision. Finally, SUs assigned to sense other PU channels perform channel switching for the next round. This scheme not only provides a feasible network set up in case there does not exist a sufficient number of SUs to satisfy the PU protection in a single round, but also offers an apparent reduction in the opportunity cost.