Is the cost for ordering multiple items sometimes, never, or always proportional

Sometimes it depends on the cost of the items if they r the same item and if the bundle like ex. say u bought 40 pack of water for 3.98. u divide 40÷3.98= 0.09 a bottle then u look at the individual cost of a bottle of water of the same brand the individual cost is 1.25. so it is cheaper to buy the case.