We propose a novel data budget-based approach to dynamically control the average power consumption of Serial RapidIO endpoint controllers in FPGAs. The key concept of the approach is to not only perform clock-gating on the FPGA-internal components of the communication controller, but to disable the multi-gigabit transceivers during idle periods. The clock synchronization, inherent to serial interfaces, enables us to omit the often needed periodic link sensing, and only enable the controller according to a predefined schedule to transmit the allocated amount of data during a specific interval. Following this approach we are able to reduce the dynamic power consumption by up to 77% on average.