In Buenos Aires you can ride the entire subway track for 0.30$, which is about 1.5 hours of subway. Without subsidies, the subway would cost at least 1$ a ride.
The hourly rate of a great part of the city workers is below 3$ and do not have full day employment.
What do these numbers mean? That a worker can get a 3.5$ an hour for 3 hours a day on the other side of the city will do it because it will cost him 0.6$ a trip. But if he had to pay 2$, that worker might not do it.
He doesn't have the choice now: he pays a tax when he buys food that forces him to take long trips to make a difference in the subway.
Overall result:
Economic efficiency: bad, because it costs more to do it this way
Welfare: bad, because it increases commute times, and reduces wages
Transit & demographics: bad because it concentrates people on the expensive side of town and increase transit exponentially