Design of seismic protective devices for highway bridges is often a highly iterative and tedious process due to the nonlinear behavior of the system, a large range of design parameters, and uncertainty of ground motions. This paper develops a design surface of system-level repair cost ratios (i.e., the repair costs normalized by bridge replacement costs) for various base-isolated bridges to facilitate the performance-based design and optimization of seismic protective devices. First, component-level fragility functions are derived for multiple base-isolation design cases as functions of earthquake input intensity. Second, nearly identical fragility functions for different isolation designs are obtained when failure probabilities are conditioned on the median engineering demand parameters (EDPs) instead. Subsequently, system-level repair cost ratio is derived by combining the failure probabilities of bridge components in terms of various EDPs. The function surface of the derived repair cost ratio with respect to EDPs is identical across different isolation designs. Hence it can serve as a performance index to facilitate the design and optimization of seismic protective devices. The proposed framework is demonstrated through a case study on Painter Street Overcrossing, a typical highway bridge in California. It is shown that optimal design parameters can be obtained to significantly reduce the overall repair cost ratio of the bridge with consideration of uncertainties and variabilities of ground motions.