By determining the optimum budget for forest fire prevention in a specific territory, an objective reference is provided of the financial cost, which tends to be high and is frequently the subject of much controversy. This paper proposes not only to establish a cost efficiency function for the fire fighting budget of a region on the basis of previous budgets, the number of fires and the average data on burnt surface area which permit marginal costs to be established; but also, the value of the natural active surface area affected, which may be assessed from two perspectives: that of the owner of the forest land - private value - and that of the public related to the forest land - private value plus public value-; this study proposes to focus on the latter. The final phase consists of comparing the marginal cost of the budget with the value per unit of surface area of the damage caused by the fires. This methodology is proposed for application to Mediterranean forest land in Spain where public values are a substantial part of the total value. As case studies we have used the forest lands of the Autonomous Community of Andalusia and the Community of Valencia.