In Manhattan, a new building of this size would cost a hell of a lot more than $1.9 billion.
This building is enormous, covering 2.9 million square feet. Google got a decent deal on a per square foot basis.
Looking at recent office new constuction in Manhattan, I don´t think Google could construct such a building for less than $3 billion.
And that assumes they want new construction. Many (most?) creative firms prefer older construction. In Manhattan, there´s definitely cachet to working in a historic building in a trendy neighborhood.
And Google needs space now, not in 5 years. They have massive expansion needs at this site.