You're reading it wrong. Here's an annotated version of what you replied to: I'd say the opposite [everything is NOT a cost/benefit analysis]. [As evidence of

You're reading it wrong. Here's an annotated version of what you replied to: I'd say the opposite [everything is NOT a cost/benefit analysis]. [As evidence of this,] We spend vast amounts of money prolonging the life of people who we know in advance are extremely unlikely to generate quantifiable benefit. How do you suppport what you've said [that everything is a cost/benefit analysis]? [The evidence shows your position to be incorrect, therefore my position that life is more valuable than cost/benefits indicate is true.]