While it's true that the price of labor, production, transportation, etc, would affect the cost of the Big Mac, it's still ultimately up to McDonalds to set the price, deciding what sort of margin they want from the product, maybe taking into account stuff like brand recognition.

So, using something like the potato would eliminate that sort of "bias" to the product, as no one party controls the potato, and thus couldn't artificially set the price higher at a whim. If the price of the potato rose, it would be because the whole market reacted to some changes to transportation or production costs, not because management has decided that they want to build their brand as being expensive and exclusive (which we know happens with a lot of brand products).