How do I sum a datagrid column after a lable function?

I need a grand total of the Avg. Cost field.  Using the code below I have an Avg. Cost per item using a label function.  SInce there is no true data coming fron the arraycollection I cannot get a grand total of the avgcost column(that I know of).
My attempt at trying to have grand total function and have it return to a text field is below.  So far I have failed at my attempts. As You will see I have tried to recalc using a function similar to the label function.

//This returns the avg.cost for each item.
 private function avgCost(item : Object, column : DataGridColumn ) : String { 
            	var total:Number;
            	var a = (item.slen * item.swid/144) * (item.ordqty)
            	var b = item.tcost;
            	total = b/a*1000;
            	
            	return currencyFormatter.format(total);
            }
 
// try to sum the avgcost column.
  public function totalavgCost():void{
var total:Number = 0
var totala:Number = 0
var totalb:Number = 0
var totalc:Number = 0
var totald:Number = 0
for(var i:int = 0; i < ac.length; i++)
{
totalb += ac.getItemAt(i).slen * pos.getItemAt(i).swid/144;
totala += ac.getItemAt(i).ordqty;
totalc += ac.getItemAt(i).tcost;
totald = (totalc*1000)/(totalb*totala);
total = totald/(ac.length);
tottxta.text = numberFormatter.format(total);
  }         
  }

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