You've purchased a car for $10,000 and now are deciding whether to have a moon roof installed for $400 and a security system installed for $200. The marginal cost of adding both the moon roof and security system is ______.

Correct Answer: $600

The marginal cost is the additional cost over and above the costs already incurred. The $10,000 price of the car should not be included in the marginal cost calculation since it is a sunk cost.