Depends on whether you have the tools needed to cook - knife + fire. If so, you can easily get <$1 per meal or even <$1 per day if you are there a few days.
Just microwave is tougher, here are some ideas:
- Eggs - <$2/dozen. Scramble with a little liquid, maybe water or you might be able to get those free half-and-half packets. I have even mixed a few of those packets with water to make "milk" for cereal, people always tell me that is gross but it tastes OK to me!
- Sweet potato - <$1/lb. Buy it in bulk and throw a few in your suitcase! Puncture a few times before you microwave, it will take like 5-10 mins depending on size & power. If you are feeling fancy you can probably get some free butter packets at the hotel, so delicious.
- Bacon - huge price range. I think even cheap microwaves usually have a "bacon" setting.
- Ham - price varies. You want the ham from the meat section, not the deli. You do not even need to microwave or heat it, just bite away. The only hard part is finding a small piece, since they are usually enormous.
- Popcorn
Definitely add some vegetables as others mentioned, at least you can get loose carrots, super cheap and you just wash and eat. Avocado is good too if you can find a ripe one at an OK price.
If you like canned spaghetti (gross), you could also try frozen ravioli, sometimes it is super cheap. I used to eat that when I was in college, like $1/bag. At least you will not exceed your daily sodium intake in one meal!