Final answer:
The costs of national security include expenses for armed sky marshals, retrofitting plane doors, and purchasing advanced security equipment for airports.
Step-by-step explanation:
The costs of national security can be extensive and can vary depending on the specific measures implemented. For example, in the case of increased airport security, some of the costs include the expenses of having armed sky marshals on every flight, which would amount to roughly $3 billion per year. Retrofitting all U.S. planes with reinforced cockpit doors would cost approximately $450 million. Additionally, purchasing more sophisticated security equipment for airports, such as three-dimensional baggage scanners and face recognition cameras, could cost around $2 billion.