http://www.retail-officespace.com/ A link that has current $/sf real estate prices in Manhattan.
The Freedom Tower, now known as One World Trade Center, will comprise $2.6 million square feet of office space at an estimated cost of $3.1 billion. Simple math produces the $/sf for the construction at $1193/ square foot.
Office space in the World Trade Center part of Manhattan rents for $80/sf on the higher end side. (per year).
1193/80 = 15 years of rent to pay back the construction cost.