Unsafe water remains a significant public health threat in high and low income countries. The World Health Organization (WHO) promotes Water Safety Plans (WSPs) as the most effective means of consistently ensuring the safety of a drinking-water supply. Although health benefits of WSP implementation have been shown, there is a lack of information relating water supplier characteristics to expected costs. Costing practices were adapted from food quality management studies and applied to six water suppliers from the Western Pacific Region, as designated by WHO. The explanation building procedure was used to develop understanding of relationships between drinking-water supply agency (DWSA) characteristics and WSP implementation costs. The results indicate that costs associated with WSP implementation are expected to be low for developed DWSAs; however, for developing DWSAs, there is high variability in costs which indicates further research may not improve generalized WSP cost estimations. Furthermore, developing DWSAs experience large and highly variable capital and operational monitoring costs. The capital and operational monitoring improvements are necessary to achieving a safe water supply and may require ongoing financial and technical resources to execute.