The adoption of good practices for the economic valuation of environmental services (ES) has strong implications in the evaluation and design of a Payment for Environmental Services program. People’s willingness to pay for an ES is useful to evaluate whether money collected from users will be enough to cover both the providers’ opportunity costs and other costs generated by the institutional arrangements required for implementation. In this article, we use a numerical certainty scale to adjust answers to a valuation question aiming to correct for hypothetical bias associated with stated preference methods. Following this approach, the mean willingness to pay decreases by approximately 70%. Values that are more conservative could assure greater political and social support for the program because more ES users would want to participate in the program; simultaneously, however, it might suggest that the project is not completely funded.