No it isn't. Eating poorly is often (much) more expensive that eating properly. Non-processed food is actually reasonably priced for the most part. Fruit and Veg is reasonably cheap. Premium cuts of meat can be expensive.
Just looking at my recent trip to the supermarket:
* A 6 pack bag of apples (Pink Ladies) was less than £1.50
* A bag of easy peeler oranges was less than £1.50
* A bag of new potatoes £2
* Large Bunch of Bananas are about ~£2
* 12 Pack of Large Eggs - ~£2.50
* 12 Rashers of Bacon - ~£2.50-4
* 1kg packet of Chicken Thighs (Raw) are ~£3.50 (2 chicken thighs a day).
* Broccoli ~£1
* Spouts ~£1
* Beans ~£1
* Berry flavoured Tea Bags ~£2
* Bag of Italian Ground Coffee ~£3-4
* 240 Pack of Yorkshire Tea Bags (will last me a month or two) ~£6
* Store brand Olive oil based Margarine ~£2
* Premium Wholemeal Loaf of Bread ~£2-3
* 4 pints of Semi Skimmed Milk £2.50
* Various table Sauces ~£1-2
That is less than £50 and I don't need to buy some of those weekly. Each day I am eating about 1800-2000 calories (I've put myself on a calorie controlled diet to lose weight) and this will last me most of the week if not the entire week.
If I buy ready meals they are £3-6 each. Premade Pizza is £4-6. So much more expensive and objectively worse for your health than cooking yourself.
One takeaway meal from the local Chinese will cost me £22 (a main and a side), Noodles will cost £7-11 depending on the dish. A pizza from the local pizza shop is £8-10. This excludes delivery. The local deli will charge me £4-5 for a meal deal (Chicken Wrap, Snack and Drink).
McDonalds is about £7 for a Big Mac Meal. KFC is similar for one of their Box Meals.