This study uses a multi-agent system to simulate the behaviour of economic players in a context of rapidly evolving environmental policy. The area under study is represented by a sector of a French overseas Department, Réunion Island, with a high concentration of pig farms in the upland region and extensive sugarcane plantations in the coastal zone. We first simulate local pig farmers' reactions to several policy options aimed at reducing the pollution coming from pig rearing. Multi-agent simulations are then coupled with cost-benefit analysis in order to calculate the net present value of different policy options. Advantages and limits of the use of the proposed methods to support decision-making are finally discussed.