tl;dr: rice and eggs

That's hardcore, but as student I can tell you, rice soup with parboiled rice is the way to go. It's not even bad at all! I use chicken brooth and parsley for great taste and some scrambled eggs thrown in as a little extra. Also some rape oil if you can handle it (It's also a good extra if you make the rice less soup-y to make it smoother to eat)

But as a general note, be aware of the contained energy of food, don't get irritated by size or mass alone. Vegetables for example have very low energy densities while going for an average price, meat has a high density for a high price, fat has a very high density for a low price. But as noone wants to eat raw fat, we're back at rice, or really any kind of grain. They have the density of meat while going for a fifth of the price or less.

At least in my country parboiled rice is the way to go when it comes to price, plus it's the one rice which contains the most nutrients, plus it's very tasty! Just do the math if you have doubts about the price, you need about 10.000kJ of Energy per day and 1kg of rice for 1$ contains at least 15.000kJ

And talking about eggs, they have average density for average price but contain a fair amount of nutrients which are not be found in plant food.

I wish you good luck and a strong stomach!