You can use the tabIndex() property of a TextInput field to set the order of tabbing.
<mx:TextInput id="amountTextInput" tabIndex="4"/>
<mx:TextInput id="refundTextInput" tabIndex="1"/>
<mx:TextInput id="priceTextInput" tabIndex="2"/>
<mx:TextInput id="costsTextInput" tabIndex="3"/>
This will go from refund >> price >> costs >> amount.
You can play around with this. The focusOut() and focusIn() events might help you too.