The Big Mac Index simplifies McDonald's into a perfectly efficient burger producing machine, in all possible countries, and then evaluates purchasing power parity between currencies by normalizing against a Big Mac.
An average burger price in a given city would be different. There the intent would be to assume profit margins are consistent (albeit perhaps not hyper-thin) from city to city, and thus get a sense of the relative costs of burger-related inputs (chiefly: labor, real estate, utilities, taxes, and transportation).
