Last I heard (on a library renovation project that crashed and burned) new construction was about $500 per square foot.
Although crudely calculated, does that suggest that rebuilding the old Penn Station would cost $500 million?
$500/SF might or might not be a reasonable place to throw the dart.
Presumably your library was a lot denser than Penn Station will be, with fancy high-tech computer systems to find the books, etc. On the other hand, Penn Station would have other high tech train and security stuff, and more of it, and presumably fancier finishes and more of them, and much more complex activities going on inside it than a library. Both will have lots of different trades working, and the pain-in-the-ass of doing Histo Presto Work for the Feds would probably drive the prices for Penn Station higher than your liberry. If your job was gonna be $500/SF, I wouldn't be surprised at $750-1000/SF for Penn Station, but The Rev. Jim is probably your best Pinhead bet for an accurate throw of this dart.
Ralph