A complex methodology has been implemented to adapt cost-benefit analysis to the interaction between rockfall hazard and mountain forests. It has been tested on six pilot areas situated in the Alps in France, Italy, and Switzerland. Starting from a precise field data collection, it combines the use of GIS software, databases and a tool developed specially to assess the economical calculation. This work allows giving an economical value for the protection function offered by the presence and the management of forests according to two different economical approaches: the substitute cost method and the damage cost avoided method. Results confirm the utility of forests against rockfall and their specific management to face this hazard. This is particularly true when: (1) the length of forest on the slope is important, (2) rockfall threatens a linear issue (roads, railways, etc.), or (3) when issues are of low economic importance. This study also shows the complementarity relevance between forest and rockfall nets. In most cases, especially when threatened issues are homes and human lives, forests cannot ensure sufficient protection, and rockfall nets or embankments are needed. However, the presence of forests allows a reduction in their size so cost is reduced.