It's cheaper to manufacture large numbers of a specific item because the variable costs are spread over a larger number of items. But consumers don't get 100% of that savings - closer to 10%-20%.
If it costs $100,000 to set up and manufacture a product, then that cost is amortized over the number of items manufactured. If you only make 1,000 of the items, then $100 of the set up cost must be added to each individual item. But if you make 100,000 of those items, then only $1 of the set up cost is added to each item.