Pre-packed vegan stuff tends to be relatively expensive, like the Amy's burritos. Convience food seems to kill the wallet.
You could occasionally cook up a large amount of whatever you like in burritos and freeze them. Bulk dry beans are extremely inexpensive, canned beans are fairly cheap and easier to work with than dried (if that's an issue). Cook up some pintos with garlic, onion, green pepper, spices--whatever you like-and mash them with a potato masher. Then fill burrito wrappers, add any thing else. Freeze them and take one with when needed, they'll save a while.
Also, oatmeal is super cheap. All you need is a bowl, oats, water, a banana, and the school microwave.. After a couple minutes..presto, excellent meal. Don't forget the spoon.