An analysis of the effectiveness of meeting the irrigation water provisions of the Leafy Green Marketing Agreement (LGMA) relative to its costs provides an approach to evaluating the cost-effectiveness of good agricultural practices that uses available data. A case example for lettuce is used to evaluate data requirements and provide a methodological example to determine the cost-effectiveness of the LGMA water quality provision. Both cost and field data on pathogen or indicator bacterial levels are difficult and expensive to obtain prospectively. Therefore, methods to use existing field and experimental data are required. Based on data from current literature and experimental studies, we calculate a cost-efficiency ratio that expresses the reduction in E. coli concentration per dollar expenditure on testing of irrigation water. With appropriate data, the same type of analysis can be extended to soil amendments and other practices and to evaluation of public benefits of practices used in production. Careful use of existing and experimental data can lead to evaluation of an expanded set of practices.