Municipal water systems in Rwanda face challenges related to infrastructure maintenance and cost-effectiveness. A Difference-in-Differences (DiD) econometric model was applied to analyse changes in water system costs and benefits over time. The DiD method compares the change in outcomes between treatment (intervention) and control groups, accounting for pre-existing trends. The DiD analysis revealed a significant reduction in water system maintenance costs by $12 per household after implementation of new management practices compared to baseline levels. The DiD model demonstrated the cost-effectiveness of implemented municipal water systems interventions in Rwanda, with substantial savings reported. Further research should explore scalability and replication of these findings across different regions of Rwanda. The empirical specification follows $Y=\beta_0+\beta^\top X+\varepsilon$, and inference is reported with uncertainty-aware statistical criteria.