Meal packs are common street food in Sri Lanka. They are also sold in small restaurants sometimes. This is the most economic option for food known to me. These are not mouth watering food per say. More akin to modest home cooked meals in decent portions. That is precisely why I like them.
Breakfast packs are 40 LKR each; contains a few dosa, idly or string hoppers with curry and sambol. The sambol is usually spicy coconut sambol. Although seni sambol (sweet) or other maldive fish sambols are sometimes sold.
Lunch packs cost 100 LKR each; contains a decent serving of rice and a few curry items like vegetable, shrimp, fish, chicken etc. Pure vegetarian lunch packs are also available. These prices are quite cheap for the quality of the food. On top of that, supporting small businesses IMHO allows one to gain some good Karma points while traveling.
The food comes wrapped in a sheet of plastic which is wrapped in a sheet of paper. You can use the plastic sheet as a make shift plate and use the paper to tidy up the place after yourself. Then neatly wrap the remains in it and to dispose off. Simple and effective.