I agree to the analogy you provided, but see the other side of the story too. What I'm saying is that developing countermeasures against ABMs are and will remain cheaper, simply because of the complexity of the problem involved. Just like tank armor vs. antitank warheads developments, this process too will never stop, and there will be always a counter developed for everything.
BMDs can best be used for protection against rogue states and unauthorized/accidental launches.
Yes, they would remain cheaper. The question is - by
how much. If there is a small difference between the costs of an evolved BM and an evolved ABM as opposed to the massive gulf in price right now, then those who field the ABM would have won a huge lead.
ABM in itself is not everything, just like SAM's are not everything. They have to be supplemented by having the ability to launch BM's of your own on short notice and the required range. One without the other is a major handicap and initself not a winning combination.
The enemy can always use
sustained saturation strikes to overwhelm even the best of SAM's like S-400, Aegis/SS/Aster. But..
The point here is three fold:
1. You force your opponent to keep upgrading and adding to his missile set just to be able to achieve what he
already could before one party fielded the SAM/ABM.
2. Economics. The weaker party gets the shaft as a SAM/ABM system
forces the opponent to upgrade/add and spend and keep spending. How long do you think he can keep matching? If he diverts funding from some other sector, that sector gets weakened. Ultimately becoming a geometric progression in inability to match the spending.
3. It gives time for your own forces to react and attack the enemy source. How long do you think it would take a determined and technologically sophisticated opponent- with their own satellite systems- to track down the source of BM's being launched against it. Minutes!
Your BM's give you that window by holding off an attack for a duation, within which you can attack and destroy the opposing BM stocks.
You make two assumptions:
You presume that ABM's would always be as costly as they are now. You are wrong, they would be drastically cheaper by another decade.
You presume that newer generation BM's designed to challenge ABM systems would be marginally more expensive than current BM's. You are again wrong. Please check out how much Bulava is costing Russia.
For an
accurate effect of installing a comprehensive ABM system, please read the analysis or account of the effect of Soviet/Russia installing SAM's and the consequences of that on UK/France and their nuclear deterrence capability/cost against the Soviet's. You would be
very surprised. This is particularly with regard to UK and their role responsibility in case of war.
This was one of the reasons why US down the line also cancelled their treaty on not having Anti-Ballistic Missile systems with Russia. There were very cold clear calculations which led to that decision.
This is a promising field, failure to invest now in this technology would have consequences a decade down the line.