Is it effective to use get; set; to avoid redundant processes?
Is it best to initialize a static member once, in a static constructor. If that is not possible, then a lazy initialization can be done like this. (Similar code to yours, just using the null coalescing operator.)

private static string file_path_includes_;

public static string file_path_includes
{
    get
    {
        return 
            file_path_includes_ ??
                (file_path_includes_ = HttpContext.Current.Server.MapPath("/_includes") + @"\");
    }
}