Good point. Similarly expensive are the Pyramids, Taj Mahal, and the Great Mosque of Isfahan.
All required public sweat and in many other cases forced labor.
But they were constructed. Without these visions and public labor very few of the world's national monuments would have existed. If they didn't exist we can't enjoy them today can we? :)
Every public project, if we think about it in the context of that time you may say that was unnecessary. Think about Lunar landing :)