Answer:
The additional cost will be total variable cost per unit.
For example assume that total variable cost per unit for the unit is $2 and the selling price of unit product is $5. The fixed cost is $100,000 and the unit produced are 35000
Now
Total cost for 35000 units = $100,000 + $2*35000 units = $170,000
Now assume that we have produced 35001 units,
Total cost for 35001 units = $100,000 + $2*35001 units = $170,002
The additional cost is $2 per unit which is Total variable cost per unit.