I'm pretty sure
a UCS report was the single most bogus "scientific" study I've ever heard of, based on my memory and what I was just able to look up with Google:
It postulated that the entire battle would have to be fought by a single space battle station (ignoring for example that the Earth revolves), and therefore a huge number of huge stations would have to be built, and that we could only loft them into space at current Space Shuttle costs (as opposed to spending a fraction of the cost on building some cost effective rockets), all of this costing "hundreds of billions of dollars" (that's the key phrase to look for, and note the dollar has lost about half its value since then).
Bottom line would be that no one denied that the UCS could design an entirely impossible strategic defense system, but the construction and destruction of such strawmen are entirely uninteresting to those who care about the truth.
Marginal cost analysis are entirely irrelevant to a discussion of the real Cold War as I noted above, the Soviets simply didn't have the money to do it, even if it would have been cheaper for them.
They'd spent several fortunes in equipping North Vietnam with three complete mechanized armies (1st used up piecemeal, 2nd crushed in their first invasion, when only 40,000 of the 150,000 men committed even made it back to the north, and of course the 3rd succeeded after the Congress stopped supplying ammo to South Vietnam), building their Strategic Rocket Forces, all their other military expenses, vast armies and air forces to invade Western Europe (including maintaining up to date lists of "future war criminals", about 10,000 for France), their vast borders guarded to keep their people in and us out, in such a way that even the rather distant and not precisely strategic territory KAL 007 overflew generated a massacre, the money they had to spend to buy grain every year they had "a bad harvest", etc. etc. etc. etc.
Combine that with things like our support of the mujahideen, our economic warfare, our moral warfare including capturing Grenada (our denial of the Brezhnev Doctrine was a shock to the system), and it was game over. Funny how after the US President to decide to actually end the Soviet Union started the effort, it went poof in one decade....