public class Order
{
  // ...
  public double Price()
  {
    return new PriceCalculator(this).Compute();
  }
}

public class PriceCalculator
{
  private double primaryBasePrice;
  private double secondaryBasePrice;
  private double tertiaryBasePrice;

  public PriceCalculator(Order order)
  {
    // Copy relevant information from the
    // order object.
  }

  public double Compute()
  {
    // Perform long computation.
  }
}