Objectives Suicide by pesticide self-poisoning is a major public health challenge in low- and middle-income countries. While effectiveness studies are required to test alternative prevention approaches, economic evidence is lacking to inform decision-making in research priority setting. Therefore, this study aimed to estimate the costs of a shop-based gatekeeper training programme for pesticide vendors seeking to prevent pesticide self-poisoning in rural Sri Lanka and assess its potential for cost-effectiveness. Methods Ex-ante cost and cost-effectiveness threshold (CET) analyses were performed from a governmental perspective based on a three-year analytic horizon, using 'no programme' as a comparator. A programme model targeting all 535 pesticide shops in the North Central Province and border areas was applied. Total programme costs (TPC) were estimated in 2019 USD using an ingredients approach and 3% annual discounting. The Sri Lankan gross domestic product per capita and life years saved were used as CET and effectiveness measure, respectively. Sensitivity analyses were performed. Results TPC were estimated at 31 603.03 USD. TPC were sensitive to cost changes of training material and equipment and the programme lifetime. The programme needs to prevent an estimated 0.23 fatal pesticide self-poisoning cases over three years to be considered cost-effective. In the sensitivity analyses, the highest number of fatal cases needed to be prevented to obtain cost-effectiveness was 4.55 over three years. Conclusions From an economic perspective, the programme has a very high potential to be cost-effective. Research assessing its effectiveness should therefore be completed, and research analysing its transferability to other settings prioritised.