NaN columnDefs

Hi, can somebody tell what I'm missing/doing wrong with the following code? The result in the front end is NaN being displayed rather than the desired result. Seem to be going round in circles trying to see what's wrong I'm sure somebody will spot it straight away and it's me being a bit slow!

{
                    targets: ['each_cost_price'],
                    render: function(data, type, row, meta) {
                        if (row['cost'] && row['unit'] && row['exrate'] !== null && data == null) {
                            return (row['cost'] / row['exrate'] / row['unit']).toFixed(2);
                        } else {
                            return (data == null) ? "" : "£" + data;
                        }
                    }
                }