to bake one loaf of bread, and the expense of heating the oven. but there is not as much extra effort and expense to stick a second loaf in the oven at the same time. after all, the oven expense is the same for two loaves as it is for one.
or imagine going to a baseball game. the cost for one person to go is the ticket price plus the cost of gas and parking and wear and tear on the car. the cost for a second person to go with the first person is just the price of the extra ticket. there is no extra gas or parking or 