Buy far the cheapest way to eat and drink while over seas is to buy at super markets, or even cheaper, open air markets.
Some Supermarket Do's
Fruits and Veggies are a great way to keep your energy
Deli Meat is prob only good in your pack for 24 hours, any longer than that, I would be thinking twice about eating
Buns for Sandwichs should last up to 3 days before going stale
Canned food Tuna, Salmon, Chicken, Turkey, SPAM (if ya like it) and many others, are non perisable and can last pretty much as long as your trip will. ( You will need a can opener)
Try to avoid items that need to be cooked, as an Urban Adventurer you can not just start a fire in the middle of the side walk to cook some beans...If your hostel is equiped with a Kitchen, check there supply of pots and pans, dishes and glasses BEFORE you go out and buy something to cook, and find they do not have what you need to cook it.
Check expery dates on food, the last thing you need is to get sick
Wine and Beer can be bought at most Supermarkets, and this is where you will get the best deal
Do not buy pre made sandwiches...one of those will cost you as much a 3 that you make yourself.
Don't go to the supermarket between 4-6:30pm, the evening crowds are crazy!!
Raw meats and Fish are a luxery you can not afford, if you end up not finding somewhere to cook it, you have just spent alot on money on something that will go bad in 10-15 hours after being taken out of the refridgerator
Salty products, Nuts, Crackers, Chips...Make you very thristy...
Junk food gives you short term enegy, try to avoid it.
Pop and Fruit Drink will end up draining you of energy in the long run, Milk, Spring Water, and REAL Fruit Juice are the better option.
Please add to this DO's and Don'ts List as you see fit, as Food is one of the bigger expences on your trip.