My 'go to' foods as a student were tuna, eggs, loads of green veg (relatively cheap), oatmeal with whey, frozen cod, tinned sardines, fruit. You don't necessarily need to blow loads of money on supps, main aim just needs to be consistently hitting a calorie surpass. Something else to factor in here is training volume: the more you train the more you'll need to eat if the aim is to add size. Be efficient with training and measure/focus on continually improving your strength.