Anonymity systems are of paramount and growing importance in communication networks. They rely on users to cooperate to the realisation of an effective anonymity service. Yet, existing systems are marred by the action of 'selfish' free-loaders, so that several cooperation incentives are being proposed. We propose a game-theoretic model of incentives in anonymity networks based on parametric utility functions, which make it flexible, adaptable and realistic. We then use the framework to analyse the cost of cooperation and the performance of the gold-star incentive scheme in the Crowds protocol.