[Goose]

The question is not the value of the defender missile versus the cost of the offensive missile, it is the value of the defender missile versus the value of what the offensive missile will hit.

BC

That question is "the question" when the defender has to decide whether or not to shoot an ABM at an incoming SRBM. The value is the net present value of the ABM. To be accurate, this value has to be multiplied by the kill probability.
 
However, "the question" when determining the number of ABMs to buy involves the actual cost of procuring the missiles, not the unknowable NPV of a particular missile shot once the war actually starts. The "goal" is to buy exactly the right number of defensive missiles. If you buy too few, at some point in the war you will "run out" and your opponent can maximize the effects of his SRBM weapons. Buy too many and you will have wasted money on weapons that you will never use.
 
When making that calculation, it is important to realize that ABMs are always more expensive than SRBMs. Relying on a plan to somehow overcome that limitation is necessarily going to fail. It is also important to consider the necessary "overkill" capacity required, if any. This estimate is important if firing multiple ABMs against incoming SRBMs is necessary. Given two opponents with equal budgets to spend, if one side selects pure defense and the other side selects pure offense, the defense will of necessity run out of missiles. Each missile costs more, which all by itself ensures this outcome. To avoid this situation, the defender will require a bigger budget. Given that an Iskander-M costs about $3 million, the Patriot PAC-3 costs between 1.3 and 2.3 times as much. That is the "multiplier". No way around it.
 
Of course, neither side will know exactly how many missiles of what type their opponents are buying. That said, defense has some significant built-in problems. The ratio is one of them. One doesn't want to be in a position of an "oops" when you launch your last ABM. You better have another way to equalize the situation.

[Goose]

The question is not the value of the defender missile versus the cost of the offensive missile, it is the value of the defender missile versus the value of what the offensive missile will hit.

BC

That question is "the question" when the defender has to decide whether or not to shoot an ABM at an incoming SRBM. The value is the net present value of the ABM. To be accurate, this value has to be multiplied by the kill probability.
 
However, "the question" when determining the number of ABMs to buy involves the actual cost of procuring the missiles, not the unknowable NPV of a particular missile shot once the war actually starts. The "goal" is to buy exactly the right number of defensive missiles. If you buy too few, at some point in the war you will "run out" and your opponent can maximize the effects of his SRBM weapons. Buy too many and you will have wasted money on weapons that you will never use.
 
When making that calculation, it is important to realize that ABMs are always more expensive than SRBMs. Relying on a plan to somehow overcome that limitation is necessarily going to fail. It is also important to consider the necessary "overkill" capacity required, if any. This estimate is important if firing multiple ABMs against incoming SRBMs is necessary. Given two opponents with equal budgets to spend, if one side selects pure defense and the other side selects pure offense, the defense will of necessity run out of missiles. Each missile costs more, which all by itself ensures this outcome. To avoid this situation, the defender will require a bigger budget. Given that an Iskander-M costs about $3 million, the Patriot PAC-3 costs between 1.3 and 2.3 times as much. That is the "multiplier". No way around it.
 
Of course, neither side will know exactly how many missiles of what type their opponents are buying. That said, defense has some significant built-in problems. The ratio is one of them. One doesn't want to be in a position of an "oops" when you launch your last ABM. You better have another way to equalize the situation.