aznriceboi17, I'm so glad you posted! This is a tough question, with a somewhat convoluted argument construction.
You're doing some excellent thinking on breaking down this argument, but I think the problem lies in what you've identified as the conclusion.
Conc: maintain respect for city authority -> preserve city hall
The author never actually says this! This seems like a potentially reasonable way to condense the two premises, but the author never actually presents this concept directly.
So, what IS the conclusion? Let's take a step back and ask ourselves what the author's overriding point it, his final logical step: it's that he disagrees with the mayor. And why does he disagree with the mayor? Because he thinks we can't afford *not* to restore city hall, while the mayor says we can't afford *to* restore it. And why does the author think we can't afford not to restore? Because of your premises above.
So, the core looks something more like this:
PREMISE: Maintaining respect for city gov't requires a sense of municipal history, and city hall is the last link to that
INTERMEDIARY CONCLUSION: We can't afford to NOT preserve city hall
CONCLUSION: The mayor is wrong when he says that we cannot afford to preserve city hall.
All the stuff about "afford" is how the author wends his way toward his final conclusion -
that the mayor is wrong. But when he says that we can't afford to let city hall wither away, he's relying on a premise about what that would cost us in terms of loss of respect for the city government and its authority. The mayor, on the other hand, is assessing what the preservation would cost in dollars and cents.
Thus, they are measuring affordability by two completely unrelated systems!
Does this help clear things up a bit?