Bus-fare collectors

Bus-fare collectors? What is bus fare?
I guess it randomly as like to collect bus card to sell at a profit.

At $170, Karen's gown cost more than many people in their town of Apopa make in a month. The $800 for the steak lunch at her party amounted to nearly half a year's wages. Their neighborhood, a warren of concrete huts set against a verdant mountain outside El Salvador's capital, is home to some of the country's poorest -- tortilla vendors, bus-fare collectors and factory workers who share the narrow streets with hungry dogs and vengeful gangsters