Note that in orthodox microeconomic theory, price is equal to the marginal value of the last exchanged unit. To use the above example of food:

> What's the value of food? If you have none you die, so the value is quit of high, but the price is much lower than that because there are many competing suppliers.

The first calories of the day, the ones that prevent you from dying, have a very high subjective value - but you pay them at the value of the 3000th calorie of the day, the extra drop of ketchup on your fries, which has a very little value.

And thus of course average value x volume is very different from (marginal value of last unit) x volume.