Edit in JSFiddle

var chart = c3.generate({
    bindto: '#chart',
    data: {
      columns: [
        ['data1', 30, 200, 100, 400, 150, 250],
        ['data2', 50, 20, 10, 40, 15, 25]
      ],
      types: {
        data2: 'bar'
      }
    },
    axis: {
      y: {
        label: {
          text: 'Y Label',
          position: 'outer-middle'
        },
        tick: {
          format: d3.format("$,") // ADD
        }
      }
    }
});
<div id="chart"></div>