So you want to find the last row added to the table, and get the value of the usd_amount column in that row?
You didn’t give us the structure of the data table, so I can’t say specifically how you can do that. But, in general, you would need to use tables.order_by to order the results by a column and then get the last row from the results.
You cannot depend on the order of results if you do not use tables.order_by to enforce an order.