In terms of cost, consider construction. Cities like Singapore, Hong Kong, Delhi, Dubai and Budapest build subways at $100-500M/km. In NYC the cost is $2,000M/km.
https://pedestrianobservations.wordpress.com/2013/06/03/comp...
I'll talk a bit about the Delhi metro since my wallet currently contains a NYC metro card and a Delhi travel card. The Delhi metro cost 10,800 crore for phase 1 and 19,131 crore for phase 2. That's about $5 billion.
http://archive.indianexpress.com/news/delhi-to-recover-entir...
http://www.delhimetrorail.com/projectsupdate/project_cost.as...
Here is a time lapse of the Delhi metro being built:
https://en.wikipedia.org/wiki/Delhi_Metro#/media/File:Delhi_...
For approximately $5 billion, Delhi got 132 miles of track (mixed subway, at-grade and elevated) and 160 stations.
In NYC, we spent $2.4 billion on the 7 line extension. We got 1 mile of subway and 1 station.
The MTA wastes virtually every dollar we give them. Giving them more money is not the solution; putting them in jail and paying Singapore to administer our transit is.