A New York Times article has some stats on the cost of subway construction in New York City compared to elsewhere:

• NYC: $2.5-3.5 billion per mile
• Paris: $0.45 billion per mile
• Madrid: $100 million per mile
• San Francisco: $1 billion per mile
• Seattle: $0.5 billion per mile

This particular article doesn’t cover costs in Asia. From my personal experience in Singapore, I know that low-cost “guest labor” is part of the equation there. And while environmental, health and safety regulations were relatively stringent there, that is certainly not the case everywhere. Still, nothing like that is going on in France or Spain to the best of my knowledge.