Of course, there are surely other reasons why burgers are cheaper than salads. These might include production costs, since harvesting apples is probably more naturally seasonal than slaughtering cows (even though both are in demand year-round). Transportation and storage costs might also play a role, as it’s probably easier to keep ground beef fresh and edible for extended periods of time, by freezing it, than cucumbers.

Why a Big Mac Costs Less Than a Salad – Economix Blog – NYTimes.com