I live on a government benefit because I am disabled (though, since discovering my gluten intolerance that's improved somewhat). So I am perminantly on a very low budget. It may not be as low as yours, I remember I was far worse off when I was studing, but I have some tips:
1. Find a safe, cheap carbohydrate. Here in New Zealand, the safe cheap carbs are rice and potatos. Rice leaves me feeling hungry, so I ate a lot of potatoes. 5kgs is 15 meals for me. And here in NZ you can get 5kgs for about $7.
2. Find a safe cheap protein. I can't eat most meats, so I basically lived on egg and potatoes, and occationally a treat of gluten-free bacon/ham (Unfortunately, just recently, I had to take out potatos from my diet because I was gaining too much weight, but unless you're obese like me, potatoes should be fine for you, and good if they're cheap where you are).
Sometimes you can get your hands on cheap meat packs, if you live somewhere near a traditional butchery, where they make their own sausages and cut their own meats, and find one who is gluten-free friendly, they will sometimes discuss how sensitive you are, and make them up for you specially. Some even use new gear so there's no cross contamination. Many of those old traditional butcheries (at least here) appreciate returning clients, and so will make the effort to keep you, espeically now days as there are so many people who are gluten free.
3. Find cheap fruit or fruit juice. If you can vary them, like here in NZ we usually have two kinds of very cheap apples, so one week I'll buy one kind of apple, and the next a different kind of apple for a change in flavours. If you add 1/3 of water to 2/3 of fruit juice you can make juice go further, this adds more nutrients to your diet and stops water being so boring all the time. If you can find one cheap fruit, and one cheap vegetable, and budget it in meal portions, you can have healthy nutrients in your diet and still be on a shoe-string budget.
4. Eating the same thing day in and day out gets boring and your body will eventually throw a tantrum for the need to have variety (I get severe sensations of hunger, depression, achiness, etc). So, what I do is throw in a little variety (it doesn't need to be a lot to make a difference). If you can't deviate much, due to finances, deviate the flavours. For example. I would do eggs, potatoes and a little cheese, then once and a while add a touch of chilli powder or curry powder (I'm sure you know to make sure both powders are gluten-free). So that it's basically the same food, but it tastes different.
And save up any spare pennies and once a month plan for a meal on a known bad week that's nice and totally different to what you're used to. For example, I have one week a month when all the bank fee's come out, and it leaves me up the crapper for bill money and food money. So I save a few dollars every other week, and buy something nice for me as a reward, like a nice pack of gluten-free bacon (which is terribly expensive here), or buy one of those lovely packs of gluten-free cookies... or whatever. If you reward yourself for tolerating the harsh conditions on occation, you'll feel a little bit better about it, and you'll have something nice to hold out for.
When I was at university, thankfully, I wasn't aware of the gluten thing at the time otherwise I might have starved, I lived on 2 minute noodles.. you know fried noodles you add hot water and flavouring to? And I ate one pack 3 days a week for two years, (the third year I got more hours at work and less classes, so I got more money). It was so boring I wanted so desperately to eat anything else, I would save up, and every few weeks I could afford to buy a little chocolate bar, or some orange juice, or a pack of eggs or whatever. And that gave me something to hold on for on the non-treat weeks. It helped me continue on without going insane and stealing from someones vegegarden in the middle of the night.
It's hard, but if you keep at it, do all you can to look after yourself, accept help when you can, and go looking for help when you can, you can make it. Just keep at it, and keep positive.
Recently, since going off gluten, I've discovered neat little shops to get some cheap gluten free things. I don't know if there are equivalent shops where you are, but take a day on the weekend and look around at the shops nearby. I found a shop that sells cheap gluten-free flours, and online, you can find good mix recipes for making certain gluten-free flours, and other foods cheaply. I make gluten-free bread by hand (you can buy all the needed flours for that fairly cheaply), my mum (who's also gluten-free), even learnt to make her own pastry flour and made gluten-free pies out of some cheap mince she'd gotten on special from the butchery!
If you're smart and do lots of research, there are many corners you can cut and money you can save. Just stay vigilant and stay positive. You'll find a way through.
Kia kaha (stand strong)!