well,OP you can start buying only fruit fresh veggies that are on sale. If bananas and apples are cheap,that's what we eat that week. If tomatoes and broccoli are cheap,then that's what I buy,and then adapt all snacks and meals to use what I buy.
Do you have an Aldi or Price Rite store near you? we are 4 people and various pets and 100.00 is about the most for a week....now keep in mind, one week it might 35.00 spent,another week 130.00- it depends on what I'm stocking up on for that week,based on need,or even better sales.
So if the chickens I want are on sale,I'll buy 6 of them to freeze. If shredded cheese is cheap,same thing,anything that can be stored to be used I buy in large amounts at good prices.
Also,USE what you buy! For instance, with the right pantry staples,you can stretch one 4 pound chicken for days for dinners.... this week,ds was sick,so I put the whole chicken in the stockpot with onions,garlic,spices- (boiled the 'guts' from the inside,fed to animals later all chopped up)
when it boiled for a while,remove whole chicken from pot,strain till it's broth,return to stove,add a bag of chopped spinach,bag of peas,bag of corn,some rice pasta and about 1/2 cup of the chicken diced fine-
put chicken in fridge till tomorrow....
next night's dinner is something I made up(as usual)
cook pasta,drain,set aside( I use quinoa)
heat about 1/4 c olive oil in a large frypan- add a bag of frozen peas(my kids LOVE peas) and a whole head of garlic,smashed,and a finely chopped onion-sautee till golden,add about 1 1/2 cup of diced chicken to sautee,about 1/2 c red wine,(sautee on high heat now)- then add pasta to pan,toss till warm,add 1/2 c grated parmesan- and serve!
3rd dinner- simple corn chowder,again, 2 bags froz. corn, about 2 cups milk,2 boxes all natural chicken stock,an onion,salt,pepper,and some shredded cheese,and what's left of that old chicken!

another cheap family fave-
That's off the top of my head,3 dinners from a 6.00 chicken for 4 people-
if you keep various staples on hand,like frozen veggies,etc,it's easy to 'stretch' your food budget....
(BTW,the chicken was more $$$ b/c it was free range organic)
I'm a big believer in frozen veggies and fruits too- BTW, couldn't regular tuna be put in a strainer,and rinsed really well under the faucet to remove a lot of sodium? Unless you're truly on a special diet,I'd think that would be helpful,and cheaper.....