In this paper, we design a pricing mechanism for downlink transmission power in cellular networks. We propose to adapt an auction-based scheme, that we have defined in a previous work for selling an infinitely divisible resource, to the context of CDMA wireless communications. The scheme aims at solving congestion problems by acting on user demand through prices, and is particularly well-suited for situations where the number of users changes over time.We establish that our mechanism is incentive compatible, i.e. the strategy that yields the highest utility to a player is to truthfully reveal her valuation for the service. The efficiency of the power allocation in terms of social welfare is also proved.