Hi OP, I feel your pain. I dont think sometimes its that healthy food costs more, I think (personally) I resent buying it more because - lets face it a banana is not as enjoyable as a 99p chicken mayo from McDonalds!
I follow slimming world and they have lots of good suggestions for people on a tight budget. Perhaps a few things you could try though are:
Having 2 meals a week that are meat free: ie, veg and noodle stir frys, vegetable pastas - so like pasta and meatballs type of sauce but with no meatballs, jacket potato and beans etc, peppers stuffed with dry packet golden rice (the white packet own brand one), eggs chips (but cut up potatos and fried not oven chips) and veg or beans.
Try the virtually fat free dry pasta sachets but shop for them and buy the fake ones in Aldi or Lidl, Its a massive filling bowl of pasta for about 39p.
Buy the cheap fruit and veg: yes fruit it expensive if you insist on only eating berry types but things like satsumas, certain apples and bananas are not that dear.
Try Aldi shopping, they highlight 5 differnt types of fruit and veg each week and make it mega cheap like a cabbage for 29p.
Honestly I seriously advise slimming world if you are looking to gain some tips and get an idea of a solid diet plan which is well balanced and substantial.