In this paper we present an approach based on game-theoretical mechanism design for dynamic spectrum allocation in cognitive radio networks. Secondary users (SU) detect when channels can be used without disrupting any primary user and try to use them opportunistically. When an SU detects a free channel, it estimates its capacity and sends the valuation of it to a central manager. The manager calculates a conflict-free allocation by implementing a truthful mechanism. The SUs have to pay for the allocation an amount which depends on the set of valuations, and they behave as benefit maximizers. We present and test two mechanisms implementing this idea which are proved to be truthful, and that are tractable and approximately efficient. We show the flexibility of these mechanisms by illustrating how they can be modified to achieve other objectives such as fairness and also how they can operate without really charging the SUs.