Bagged coleslaw - Foodstuffs supermarkets have a great one that’s shredded beetroot and $4.50. Doesn’t come with dressing. Add boiled eggs and Mayo and it’s a meal. You’ll get three meals out of one if it’s just you.
There’s also two brands of coleslaw for about $6 depending on the supermarket that are good. VitalSlaw is the one I get most often.
Add a chicken breast if you feel like cooking, which I regularly don’t so I use a $3 tin of tuna (the 185g one)
I also use the markets a lot for greens, but not everyone can. $20 worth of kale, spinach, Choi sum and Kai laan lasts me a week. I cook it in advance. My food budget is $20 per day. I eat twice the RDI of calories, so $10 is doable. The kicker is you have to actually cook; prepared food is expensive.