Feeding troops in Iraq is expensive, as the food is generally of the same variety and quality found back home, but shipped in from regional and American suppliers, and delivered fresh (or frozen or chilled). To do this for the 140,000 or so American military personnel in Iraq costs about $18 per day per person. This covers warehousing and delivering some 50,000 different food items. Cooking and serving it costs extra.
It's a bit more expensive in Afghanistan, because the truck routes are longer and more subject to tribes seeking either a bribe, or a taste.