You can't negotiate cost
There is a common misunderstanding about the difference between price and cost that many businesspeople still harbor. Profit is the difference between cost and price, and in the process of arriving at an estimate, some people will try to negotiate cost. Cost, however, is a constant and can't be negotiated. By analogy, a car buyer might reasonably negotiate with the salesman over the sticker price, but not over the dealer's cost. Trying to negotiate cost when making estimates turns the endeavor into a game. Instead of padding cost estimates, a better strategy is to use a range-estimating process and emphasize to stakeholders that the estimate is a cost range, not a price range.