Usually when people buy or use more quantity of a given product, they expect the price per item to reduce or stay the same. I buy a Gold bar today,...

Usually when people buy or use more quantity of a given product, they expect the price per item to reduce or stay the same. E.g.: I buy a Gold bar today, the rate is USD $13,510. If I buy 5 gold bars it would be $13,510 x 5 = $67,550

In the above example, it would mean that 1 gold bar costs USD $13,510 but 5 gold cars would costs more than $67,550.  Please explain what factors would cause the price-volume rule to not be obeyed.