Eating junk is really expensive! I don't understand how students eat ready meals and takeaways because I would never have had enough money to do that, even if I'd wanted to. I think I've probably had takeaway pizza less than 10 times in my life (and I'm in my mid-20s now) because I can't justify the expense when I can make a better pizza for far less money.
That said, eating really healthily isn't cheap either. The cheapest way to eat is to fill up on loads of inexpensive carbs - pasta, rice, bread, potatoes. This required a basic level of cooking skill beyond just shoving things in the microwave, but is definitely not healthy eating either.
A lot of people think eating healthily means eating loads of fruit, but that isn't really necessary. Fresh fruit is expensive (especially nice fruit, not just apples and bananas), and it's also high in natural sugars so isn't great for your teeth. You can get most of your vitamins from vegetables which are often cheaper and lower in sugar.
I would say the best way to manage a healthy diet on a budget is:
-more vegetarian meals, particularly using beans/chickpeas/lentils. These are filling and a source of protein and fibre and are also cheap.
-use a variety of veg (buy things on offer or from a market). Keep an eye on the prices for loose veg by weight - they often fluctuate based on what's in season, even in the supermarket.
-buy meat and fish when reduced to clear - check the reduced section every time you go to the supermarket and buy anything that's a good price and freezable. Then plan your meals based on what you have in the freezer. For example, I currently have loads of lean beef mince, some lean pork mince, some chicken breast, some venison burgers, some salmon, some diced lamb and some squid. These all cost less than half of their normal price because I bought them on their sell-by date.