The Friday File: While the cost of a plain 16oz cup of coffee at Dunkin’ Donuts, McDonald’s, Starbucks and 7-Eleven ranges from roughly $1.20 to $2.65, caffeine content/cup also varies, specifically, from a low of almost 150mg/cup to almost 350mg/cup. Now, comparing milligrams of coffee per dollar, the best value is 7-Eleven at 129mg/$1, followed by Starbucks at 125mg/$1, McDonald’s at 122mg/$1, and Dunkin Donuts at 100mg/$1.