You could get a ballpark figure by looking at Crossrail: http://en.wikipedia.org/wiki/Crossrail http://en.wikipedia.org/wiki/Crossrail 73 miles of track, 40 st

You could get a ballpark figure by looking at Crossrail: http://en.wikipedia.org/wiki/Crossrail http://en.wikipedia.org/wiki/Crossrail 73 miles of track, 40 stations, 26 miles of tunnel under London. £16bn ($27bn-ish). So, for your project, I'd handwave at $1-5bn.