The systematic use of landfills as a method of waste disposal is leaving a heavy legacy in terms of environmental pollution, especially in the case of illegal or non-compliant disposal sites. When no environmental precaution is adopted, harmful substances’ release imposes relevant health risk to the surrounding population. This risk is also reflected on the value of nearby assets. To mitigate this pressing problem, the European Union is planning and financing the remediation of thousands of contaminated sites on its territory. Cost–benefit analysis (CBA) is used for assessing the priority of individual projects and for accessing European funding, evaluating the effectiveness of an investment considering the externalities generated. In this paper, we apply CBA to two emergency securing interventions of contaminated sites in Southern Italy. We execute a financial analysis and an economic evaluation of four environmental and social spillovers (reduction of mortality, recovery of real estate and land assets values and landscape enhancement). This study contributes to the literature of CBA applied to environmental projects, proposing a systematic and replicable method for evaluating externalities generated by contaminated sites’ emergency securing. Being based on two real case studies, this work allows to compare similar interventions, highlighting the most effective action strategy.