Crowdsourcing is an online activity in which an individual, an institution, a non-profit organization, or company proposes to a group of individuals of varying knowledge, heterogeneity, and number, via a flexible open call, the voluntary undertaking of a task. Crowdsourcing has been traditionally considered suitable to provide different types of support to the decision making process, especially in the design phase, through idea generation and co-creation, in the choice phase, through voting, as well as in the intelligence phase to explore or exploit information about the issue to be investigated. This article aims to investigate how to perform scenario planning by exploring ways to use crowdsourcing as a complement to two standard techniques for idea generation and selection: (a) brainstorming and (b) the Delphi method. Then, we question the cost and the effectiveness of combining these methods, and crowdsourcing to perform scenario planning for policy making. To this end, in this article we propose a model to assess the cost and effectiveness of the intersection between crowd and experts in decision-making activities, with a focus on scenario planning, choosing a public sector research site for its evaluation.