Order.cs

public class Order
{
    private readonly List<OrderLine> _lines;

    public Order()
    {
        _lines = new List<OrderLine>();
    }

    public decimal Amount
    {
        get { return _lines.Sum(x => x.Price); }
    }

    public void AddLine(decimal price, Product product)
    {
        _lines.Add(new OrderLine(price, product));
    }
}

public class OrderLine
{
    public decimal Price { get; private set; }
    public Product Product { get; private set; }

    public OrderLine(decimal price, Product product)
    {
        Price = price;
        Product = product;
    }
}