I would copy the AMT to the table where you specify the quantity and multiple the QTY by AMT there record by record so that a sum can be computed. That way, a future price increase won't result in past transactions recalculating to use a new AMT value--which would show incorrect totals on those past transactions.
But to do what you describe:
AMT * Sum ( RelatedTable::Qty )
To do what I recommend:
Sum ( RelatedTable::ItemCost )
where ItemCost is defined as: AMT * Qty and AMT uses an auto-enter field option to copy over the current AMT from the other related table.
Thank you very much!