About $400 a month for 3 (2 adults and one 5-year-old).
I’m an Atkins adherent (I keep it at about 45 carbs maximum a day) and my wife is a South Beach adherent. We’ve been that way for about five years. Our daughter is loosely South Beach.
Naturally, we eat very few carbs and starches, so we can’t get away with buying cereal or Ramen noodles to save money.
Most of the money goes out for meat, dairy (LOTS of milk!), and greens. LOTS of coffee, too. Maybe $75 a month goes out for coffee, heavy whipping cream (it has 0 carbs and makes a great creamer for coffee), and Splenda. About $30 a month goes out for CarbSmart ice cream as our after-dinner dessert of choice.
Hope this helps.
Cheers
P.S. We use multivitamins, but not religiously. Maybe once or twice a week. More if we’re feeling run down or fighting a bug.