using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var items = new List<Item>

        {

            new Item{ Id = 1, Name = "Machine" },

            new Item{ Id = 3, Id_Parent = 1,  Name = "Machine1"},

            new Item{ Id = 5, Id_Parent = 3,  Name = "Machine1-A", Number = 2, Price = 10 },

            new Item{ Id = 9, Id_Parent = 3,  Name = "Machine1-B", Number = 4, Price = 11 },

            new Item{ Id = 100,  Name = "Item" } ,

            new Item{ Id = 112,  Id_Parent = 100, Name = "Item1", Number = 5, Price = 55 }

        };

        foreach(var item in items)

        {

            Console.WriteLine("{0} {1} $" + GetSum(items, item.Id).ToString(), item.Name, item.Id);

        }

    }

    public static int GetSum(IEnumerable<Item> items, int id)

    {

        // add all matching items

        var itemsToSum = items.Where(i => i.Id == id).ToList();

        var oldCount = 0;

        var currentCount = itemsToSum.Count();

        // it nothing was added we skip the while

        while (currentCount != oldCount)

        {

            oldCount = currentCount;

            // find all matching items except the ones already in the list

            var matchedItems = items

                .Join(itemsToSum, item => item.Id_Parent, sum => sum.Id, (item, sum) => item)

                .Except(itemsToSum)

                .ToList();

            itemsToSum.AddRange(matchedItems);

            currentCount = itemsToSum.Count;

        }

        return itemsToSum.Sum(i => i.Price);

    }

    public class Item

    {

        public int Id { get; set; }

        public int Id_Parent { get; set; }

        public int Number { get; set; }

        public int Price { get; set; }

        public string Name { get; set; }

    }

}