Building-integrated greenery (BIG) systems, which include green roofs and green facades, are well-established nature-based solutions (NBS) with proven scientific benefits. However, initial costs and economic apprehensions stemming from potential negative outcomes act as adoption barriers. Furthermore, the lack of standardized indicators and assessment methodologies for evaluating the city-level impacts of BIG systems presents challenges for investors and policy makers. This paper addresses these issues by presenting a comprehensive set of indicators derived from widely accepted frameworks, such as the Common International Classification of Ecosystem Services (CICES) and the NBS impact evaluation handbook. These indicators contribute to the creation of a ‘sustainability factor’, which facilitates cost–benefit analyses for BIG projects using locally sourced data. The practical application of this factor to a 3500 m2 green roof in Lleida, Catalonia (Spain) demonstrates that allocating space for urban horticultural production (i.e., food production), CO2 capture, and creating new recreational areas produces benefits that outweigh the costs by a factor value of nine during the operational phase of the green roof. This cost–benefit analysis provides critical insights for investment decisions and public policies, especially considering the significant benefits at the city level associated with the implementation of BIG systems.