THAAD is what you deploy to a place like South Korea where that 200km radius covers the entire country. For protecting something like the US mainland, you would use GMD, which can cover large sections of the continent.
But even if you did just have to build large numbers of short range interceptors, that's still perfectly acceptable. Likewise for upkeep and maintenance. You don't need to protect everything, you only need to protect the targets that justify the cost.
But even if you did just have to build large numbers of short range interceptors, that's still perfectly acceptable. Likewise for upkeep and maintenance. You don't need to protect everything, you only need to protect the targets that justify the cost.