The present article estimates the financial benefits of water reuse by calculating the annualised total cost of water and wastewater management, using mixed integer linear programming. The programme is using as input: geographical data, population distribution, and groundwater availability (for a given area), to calculate the qualitative localised water needs, and to estimate the sizes and locations of water and wastewater management infrastructure, so as to minimise the relative annualised total cost (capital and operating). The programme is used to calculate the optimum water and wastewater infrastructure (and the relative annualised water and wastewater management cost), with and without the option for water reclamation and reuse. One case study is presented for the Greek islands of Santorini and Thirasia. The proposed model has showed significant computational benefit, compared with previous models. Thus, for Santorini–Thirasia Islands, the total annualised cost for optimum water and wastewater management infrastructures, with water reclamation, has been calculated as $2,153,694, while the above cost has been calculated as 19% higher, if water reclamation is not an option. It is obvious from the computational results that water reuse can reduce significantly the total water and wastewater management cost.