Hmm... interesting premise but I recently came to the conclusion that factoring water and fibre content out any analysis of my nutrition is a good idea. My reasoning is:
1. Water - I am incredibly unlikely to get too much from food/food ingredients alone, so drinking water (including teas), water added to food (soups, drinks), and amount of water extracted from fruit and vegetables (juice) can be adjusted to give me optimal water intake. Adjustment is very cheap.
2. Fibre - by eating mostly whole foods and fruits and vegetables, it is almost impossible to get too little fibre, and getting a little bit too much (up to 3-5x RDI) seems to pose no real problems. Getting a LOT too much is also pretty unlikely, unless you really plan your diet hard in that direction.
3. Everything else - micronutrients (vitamins, minerals and desirable optionals like particular antioxidants or possibly enzymes) and caloric nutrients/macronutrients (various types of fats, carbohydrates and aminos). These are the things I might have too much or too little of, and the things that are going to be a cost factor in my diet. I figure I can get calories/kilojoules pretty cheaply, of carbs, fats, or even protein, so it is not the food cost per kilojoule that interests me, or even macronutrient ratios as much as the amount of each nutrient per kilojoule and per dollar.
If I feel the need to grossly oversimplify things, I can classify foods as nutrient cheap or nutrient expensive (or somewhere in between) and, orthogonal to this, as energy dense or nutrient dense (or somewhere in between).
Under this reasoning, I am not sure popcorn looks so relatively impressive any more... not that bad, but maybe not that good either... by all means eat some if you feel like it.
