"Monks" are not one, monolithic community, so it's hard to argue one way or another whether they contribute anything meaningful back to a community. Here in Japan, many Buddhist orders are downright wealthy (from donations), and use their considerable resources to preserve traditions and history. I doubt that you can put a dollar value on the benefits of keeping cultural history alive. In a sense, it's not too much different from the function of the Metropolitan Museum in New York. Of course, Buddhism is a many-facted religion that manifests differently in different places.