Here is an example for enumerating the datagrid. You can easily change it to do selected items instead. You could also access the dataprovider source directly vs going through datagrid.
protected function UpdateTotal():void
var fRunningTotal:Number = 0.0;
var nNodeCount:int = 0;
//Check if the dataprovider is empty.
if (dgMyDatagrid.dataProviderLength == 0)
//Nothing to total, return out.
//Loop through all the nodes
for (nCount = 0; nCount < dgMyDatagrid.dataProviderLength; nCount++)
//At the current row in the dataprovider convert the 5th object (0 index = start).
//Add to the running total.
//If the dataprovider is an XMLListCollection you could address it by name.
fRunningTotal += Number(dgMyDatagrid.dataProvider.getItemAt(nCount))
//Do something with the total here... like store it for displaying in a label.
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