Format Highcharts y-axis labels

You can call the original formatter from your formatter function:

$(function () {
    $('#container').highcharts({

        yAxis: {            
            labels: {
                formatter: function () {
                    return '$' + this.axis.defaultLabelFormatter.call(this);
                }            
            }
        },

        series: [{
            data: [15000, 20000, 30000]
        }]

    });
});

http://jsfiddle.net/x6b0onkp/2/