Wireless access based on slotted Aloha with selfish users may result in very inefficient use of the system resources. To impose cooperation and fairness in such systems, we propose an optimal pricing strategy, based on which the service provider can regulate the overall network behavior. As the users' utility incorporates the price paid for using the spectrum, by striving to improve their own performance, the users act to optimize the overall network performance. Our analysis is based on a game theoretic framework, and we consider both the simple collision model for packet reception, as well as multipacket reception capabilities for the physical layer. The proposed pricing strategy enforces fairness under the constraint of an equal access probability.