Shared network testbeds, like PlanetLab, rely on the individual contributions of participants in order to build an infrastructure of sufficient size to allow them to run large scale experiments on overlay networks created over the Internet. In this position paper, we study the design of incentive mechanisms for encouraging resource provision and allocating resources efficiently upon contention. We analyze the economic characteristics of shared network testbeds and argue in favor of a rule-based mechanism relating contribution with allocation as a realistic and effective way to increase the system’s efficiency. We support our reasoning based both on theoretical, economic, arguments and available PlanetLab usage data. We also make a first step towards the design of a practical rule of this kind by analyzing its main attributes: the definition of a site’s effective contribution and the different levels of service offered by the system.