Hi Randall,
The default Estimate screen uses calculations on the line items (for example to calculate the amount), so from the sounds of your request this is where you would add additional actions for the extended cost. To get to these actions, first open up the Estimate screen for editing (making a copy if you haven't customized the default screen yet). Then, click on Edit on the Line Items grid. On step 2 of the wizard, click Actions beside item. Then select Text Change on the Event dropdown.
All of the default calculations happen on the Text Changed event of the Item dropdown, so you could add your custom calculations here as well. You would first need to add your Extended Cost column to the grid, then get/set it's value using the necessary actions.
You also have the option of looping through the grid or table it is attached to (in this case EstimateLine), if that works better for your workflow.
You could involve a Method Consultant to help you with this project (http://www.method.me/support/assisted-services/). If you would like to explore this possibility please speak with your account manager.
If you have any quetions about this just let me know.
Regards,
Russell