What’s the most that a restaurant should charge for a Coke?

Soft drinks are like concessions for restaurants because our addiction to a bit of colored corn syrup can help an establishment’s struggling profit margins. But the line between gouging and gorging is a fine one.
Serious Eats recently stumbled into a $5 bottle of Mexican Coca-Cola at David Chang’s Duck Lunch in New York City. (Last week, chef Alex Pope talked about the morning he spent at Chang’s Momofuku.) That’s $5 for a bottle that you carry from the counter to your table. Stadiums, bars and airports are exempt from the discussion, but what’s the most that a restaurant should charge for a Coke?