This is the part that I'm pretty sure Goldman Sachs hasn't accounted for. That whole "could cost up to $140Bn" thing sounds very inflated.
Look at it this way. According to them, if Google spends 1/4 of its $4.5Bn it could equip 0.83million homes. That's $1.125Bn for 0.83million homes.
On the other hand, Verizon has spent $15bn and equipped 17million homes. For google to pass 17million homes, using the above calculations, it would cost them $25bn. Those numbers don't make a lot of sense to me.
I think the difference in these numbers is "nationwide" and what it might mean. Google could probably easily equip the same number of subscribers as Verizon with fiber for the same cost or less; however, there are some middle-of-nowhere places that would require a much more significant expenditure per potential customer than the places Verizon has deployed. Maybe to run a line to everyone everywhere costs $140 billion, but to reach, say, 30% of the population would cost $25 billion.