The Santiago metro is the most expensive subway in Latin America. At 740 pesos it is the only one over a dollar (current exchange is about 709 pesos to the dollar).
This has obvious socio-economic implications because the cost of transport cuts into wages, and people may have to take multiple sources of public transportation (metro, bus, and/or colectivo, which is like a group taxi). I would guess Santiago is more expensive primarily because it is subsidized less. On the other hand, Mexico City's highly subsidized system is quite cheap but faces serious problems of infrastructure neglect.