In this paper, we consider the problem of pricing the spectrum usage in a cognitive radio network. In such a network, where licensed/primary users (who has the right to use the spectrum) and the unlicensed/secondary users coexist, a secondary spectrum market can be established where the primary network service provider charges the secondary users for the usage of the spectrum, while the secondary users also compete with each other for the access. Several important questions arise regarding the operation of such spectrum market: 1) how the spectrum should be priced; 2) how the secondary users should distributedly access the spectrum based on the spectrum price. To answer these questions, we interpret the available spectrum in the network as the aggregated interference tolerable at the receivers of the primary users, and introduce the notion of a market equilibrium in which the prices of the spectrum/interference are set correctly such that 1) the supply of the spectrum equals the demand of the spectrum, and 2) the secondary network is stable. We also developed an algorithm to distributedly compute such market equilibrium, and proved its convergence.