code: public Number getCostVariance()
     {
        Number variance = (Number) getCachedValue(TaskField.COST_VARIANCE);
        if (variance == null)
        {
           Number cost = getCost();
           Number baselineCost = getBaselineCost();
           if (cost != null && baselineCost != null)
           {
              variance = NumberHelper.getDouble(cost.doubleValue() - baselineCost.doubleValue());
              set(TaskField.COST_VARIANCE, variance);
           }
        }
        return (variance);
     }
docs: The Cost Variance field shows the difference between the baseline cost
  and total cost for a task. The total cost is the current estimate of costs
  based on actual costs and remaining costs.
  
  @return amount