I stick with salad and protein for lunch.
salad: usually a mix of spinach and greens, grated carrot, cucumber, radishes, cabbage, and other veggies that are available (and affordable)
protein: I choose from one of: steak, chicken breasts grilled, canned salmon, tuna, cottage cheese, feta cheese, and lowfat string cheese.
Thats about 300 calories for a big meal!
Don't forget about soups, I know its not soup season but they have great flavor and some are very low calorie.
I don't buy many pre-packaged meals, but I do have high protein slimfast handy in case I can't cook or get stuck at work.