Cognitive radio is an enabling technology that allows unlicensed users to opportunistically access the spectrum in order to enhance the spectrum efficiency. In this paper, we consider a cognitive system wherein there exists a primary relay network and a secondary network. In order to efficiently exploit the available spectrum and gain revenues whenever the primary relay infrastructure is not utilized, the primary network leases its unused bandwidth and the idle relay node to the secondary users. As a reimbursement, the secondary users make payments to the primary network based on the service they receive. We first characterize the interactions between the primary and secondary users using a buyer/seller model. Specifically, the price is determined by the primary network such that the revenue is maximized. On the buyer side, given the specified price, the secondary users competitively access the spectrum and employ the primary relay node to forward their packets. Then, we model each...