The zone diet, with a 3-block meal (21 grams protein, 27 grams carb, 9 grams fat) is a good balance. A smaller meal would be 14 grams protein, 18 grams carb, 3 grams fat). A very inexpensive way to do this is a hard-boiled egg and about half a slice of bread, or two eggs and one slice. And maybe a teaspoon of mayonnaise.
You can also use canned or packaged fish, such as tuna or sardines. Go for packed in water or mustard or vinegar, but not packaged in oil, as that is too much fat. If you can afford it, sliced lunch meat - preferably low fat, like turkey. If you're vegetarian, tofu is a good choice.
For carbs, you could replace the bread with rice, or under 1/2 cup of pasta or potato salad (vinegar, not mayo). Or you can go with lots of cooked or fresh vegetables, spiced to taste, instead of bread, rice, or pasta.
Cottage cheese and apple sauce, or low-fat hard cheese and an apple are a good snack or supplement option.
Adding salad greens (as much as you want) or cooked vegetables with spices, lemon, or a water (not oil) based dressing is good, too. A piece of fruit is a good idea.
And if you need it, a bit more oil or butter is okay. To learn more about the Zone diet, you can read my hub Is There a Right Way to Eat, or look up books by Dr. Barry Sears.