Cognitive radio (CR) has been introduced to accommodate the steady increment in the spectrum demand. In CR networks, unlicensed users, which are referred to as secondary users (SUs), are allowed to dynamically access the frequency bands when licensed users which are referred to as primary users (PUs) are inactive. One of the most important issues in CR networks is how to share the spectrum effectively among the different users which is denoted as spectrum trading. Spectrum trading aims to satisfy the objectives of both types of users (i.e. PUs and SUs) by balancing these overlapping objectives. In this paper, we propose a noncooperative game theoretic model to allow PUs to gain high profit from renting their unused frequency channels to SUs which use proper power levels over these channels for their data transmission. The proposed model will finally converge to Nash equilibrium (NE) by following the best response dynamics. Choosing the best response strategy by each game player (i.e. PU and SU) based on the perceived opponent strategies is also shown in this paper. Simulation results show that the proposed model allows PUs to make extra revenue and satisfy the quality of service (QoS) of SUs.