It depends on where you want to access it.

If you want a 4th column to display Cost * ProcessTime, then you could create an ItemRenderer/GridItemRenderer which takes the data from the column in order to get the values

i.e.  In the GridItemRenderer

override public function set data(value:Object):void {

     super.data = value;

     if(value != null) {

          itemRenderer_Label.text = Number(data["Cost"]  *  data[""] ).toString();

     }

}

Or, you could just add it when you populate late the array.

Then you should add it to the Array when you populate it and make sure you have a unique identifier for the row.  If "XXX" is the product name, when you do the insert, you need to add a column for the unique ID.

I handle my ArrayCollections by creating an Object class then inserting each Object into an ArrayCollection.  I usually access my data from a webservice which provides the XML data to populate the ArrayCollection.  Once the ArrayCollection is populated, then I can simply access the row and then the object column to get the data that I need.

i.e.

Add each item to the Array Collection

myArrayCollection.addItem(ProductObjectClass.addItem(ProcessName,Cost, ProcessTime));

When you want to get the information then you can use  myArrayCollection.getItemAt(i) to put it back into an ProjectObjectClass and find the correct value within that object;  Where i would be the row.  If you click on the DataGrid, then you would have the rowIndex value to get that rows data with the unique id.  Then when you did an update, you could update based on uid.   Or, you could have inline editing on the DataGrid.

public class ProjectObjectClass extends Object

{

public static function addItem(thisProcessName:String, thisCost:String, thisProcessTime:String):Object{

     var now:Date = new Date(); // create date time instance in order to use time stampe as unique id

     var poc:ProjectObjectClass = new ProjectObjectClass();

     poc.uid = now.valueOf();

     poc.processName = thisProcessName;

     poc.cost = new Number(thisCost);

     poc.processTime = new Number(thisProcessTime);

     poc.processTimeCost = poc.processTime * poc.cost;

     return poc

}

public function ProjectObjectClass()

{

     super();

}

  [Bindable]

  public var uid:Number  

  [Bindable]

  public var processName:String;

  [Bindable]

  public var cost:Number;

  [Bindable]

  public var processTime:Number;

  [Bindable]

  public var processTimeCost:Number;

}