Depends on the cereal. kellogs is just junk food of course, but you can buy oats, crushed wheat etc in a raw form.
https://www.wolframalpha.com/input?i=rolled+oats+200g+%2B+50...
(Plus a bit of dried fruit, but I'm not sure how much, this far removed).
That's just two full bowls. Dinner was usually just one pack of instant noodles from Lidl.
At the time, this cost me about £0.50/day.
the key was that this meal lasted me for 24 hours. so practically only one meal a day.
i don't know how long i did that, or how frequent. i doubt i would have done that everyday for more than a few weeks. more like weekdays with some other meals in between once in a while. but i really have no memory of that aspect of it.
I made a game out of how little I could spend without feeling hungry, so I wouldn't recommend it to anyone for longer than a university term.
The peril of editing without checking afterwards. Synonymous with the adjective I used? something like that.
This has more protein than pure oats, but you're right the calorie density is a problem. A bowl of oats cooked as porridge has about half to 2/3rd the calories of a bowl of cooked pasta, depending on how thick you make it. I usually add some butter or olive oil to improve this.
Than, last year, a work colleague turned me on to the idea of stirring in peanut butter. It works surprisingly well and tastes good (particularly with banana). I never found porridge particularly filling until I started doing this.
The Horse, The Phil Silvers Show S1E4