Green infrastructure is increasingly incorporated into the existing urban drainage system, i.e., grey infrastructure, for stormwater management in the context of rapid urbanization and climate change. Whether the coupled green–grey system can yield additional benefits requires quantification of environmental and economic performance. This study proposes a cost-combined life cycle assessment method to evaluate the environmental and economic benefits of coupled grey and green infrastructure system. Compared to traditional assessment methods, this method introduces internal (e.g., material cost, energy cost) and external (i.e., human health, ecological restoration, and pollutants emission) economic costs to conduct assessment. This research identifies key factors (i.e., categories, processes, and substances) of a coupled grey and green infrastructure system in a typical residential area in China. The coupled system is shown to generate higher environmental impacts and economic costs than grey system in the construction stage. The operation stage of coupled system can observe significant environmental and economic benefits. The payback time of related environmental impacts (except for non-carcinogens) and total economic costs is 4 years for the current case study. Noteworthy, a currently existing subsidy scheme would allow to reduce the payback time of total economic costs from 3.78 to 1.18 years. This study provides concrete evidence in support of urban stormwater management.