How do I display the sum of two Text Inputs on the fly in Flex 3?

Hello,

I can accomplish the following if a button was clicked or if I initiated it with a labelfunction (in a datagrid for example), but I can't figure out how to do this on the fly...

What I am trying to do is that when a user enters a Product Cost in one TextInput and the Taxes in another TextInput, I would like to have a label beneath that calculates the running total as the values are entered in...

Any insight would be greatly appreciated!!!!!!!  Thank you!!!!

<mx:Form width="100%" height="100%"   >
		          <mx:FormItem label="Total Product/Service Cost: $">
		                <mx:TextInput  id="Product_Cost" />
		          </mx:FormItem>
		          <mx:FormItem label="Taxes total: $" >
		                <mx:TextInput id="Taxes" />
		          </mx:FormItem>
		          <mx:FormItem label="Total: $" >

<!--this label obviously doesn't work, but the text is what I am trying to accomplish just in case my question wasn't clear enough.  Once again, THANK YOU!!!!!!!-->		                
<mx:Label id="Total"  text="{Product_Cost.text + Taxes.text}"/>
		          </mx:FormItem>
		          
		       </mx:Form>

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