Towers of ilium was not privy to the decision making process for the 2nd Avenue subway system in New York, but reports of its cost are a matter of public record. Those costs, incidentally, are quite high--somewhere north of 14 billion when it's fully complete in about five years. A design feature that drives those costs is the double height stations. A group of people thought this was not just a nice idea, but an essential idea. Now, design costs and initial construction costs are completely sunk.
No one asked the question: Will the average commuter give a damn?