var expr =
  from c in customers
  join o in (
      from c in customers
        from o in c.Orders
        join p in products
            on o.IdProduct equals p.IdProduct
        select new { c.Name, OrderAmount = o.Quantity * p.Price }
      ) on c.Name equals o.Name
      into customersWithOrders
  select new { c.Name,
            TotalAmount = customersWithOrders.Sum(o => o.OrderAmount) };
