Where the extra cents come from
Divide preparation evenly across ten parts and the arithmetic looks fine — until the last plate carries two parts instead of four and still costs a whole setup. The per-unit price of an underfilled run is genuinely higher, and the calculator says by how much.
This is the honest argument for adding two more parts to a plate rather than a vague feeling that bigger runs are cheaper.
What a batch does not change
Filament, electricity and wear scale with the parts and stay the same per unit. Discounts on volume come from your labour and your plates, not from the plastic — which is why a wholesale price built on "it is cheaper in bulk" alone tends to lose money.