This paper proposed a net benefit value (NBV) model for cost–benefit evaluation of wastewater treatment and reuse projects, and attention was mainly paid to decentralized systems which are drawing wide interests all over the world especially in the water-deficient countries and regions. In the NBV model, all the factors related to project costs are monetary ones which can be calculated by using traditional methods, while many of the factors related to project benefits are non-monetary ones which need sophisticated methods for monetization. In this regard, the authors elaborated several methods for monetization of the benefits from wastewater discharge reduction, local environment improvement, and human health protection. The proposed model and methods were applied for the cost–benefit evaluation of a decentralized water reclamation and reuse project in a newly developed residential area in Xi'an, China. The system with dual-pipe collection and grey water treatment and reuse was found to be economically ineligible (NBV > 0) when all the treated water is reused for artificial pond replenishment, gardening and other non-potable purposes by taking into account the benefit of water saving. As environmental benefits are further considered, the economic advantage of the project is more significant.