In this paper we propose a novel sequential score fusion strategy for multi-biometric systems. The strategy's aim is to reduce the cost of a multi-biometric system by dynamically fusing the optimal number of systems required to take the final decision. This way we optimise at the same time cost and performance in the system. The novelty of this paper lies in the automatic tuning of the decision parameters (thresholds) at a desired level of performance by revisiting the Sequential Probability Ratio Test (SPRT).