I made raised beds from pallets got free from local builders yard. Cut the ends off and lay on side (so they look like a long rectangle laid out flat. You get three of these usually per pallet. Then cheapo L Brackets in each corner. You can plant smaller things, onion or herbs in the little planters that laying them out like this makes and the rest in the middle.
I grow garlic as a perennial so I get lots of leaves and scapes to eat each year, and if I want bulbs could dig some up but I don't tend to bother. I also have onions which I left to flower the next year and they then self seed. I use these as 'spring onions' in that I just pull the green leaves - never stripping one plant - and they seem to just go on and on.
Things like a looser leaf cabbage or a kale are good value because Kale is stupidly expensive and very productive in the garden. Never pull all the leaves from one plant - just get a one or two from each if you have a few - and they will go on and on until they flower, at which point you snap off the flower heads and eat them like sprouting broccoli. The plant will go on and on giving these flower heads for ages. My best cut and come again cabbage is from Real Seeds and is called Becky and Paul's Asturian Tree Cabbage. We've been eating it all autumn and winter and it's just started with the flowers.
Potatoes etc as others say are not cheaper though much tastier grown at home.
I have lots of herby leafy things which are perennial and look after themselves and mean I can not buy salad leaves most of the spring, summer and some of the autumn. Salad Bennet, French Sorrel, Nasturtiums (not perennial but self seeding), Stridolo are my main go tos for this.
I make my own compost, and buy manure cheap from local horse place.
Beetroot grow very easily and give a good return.
I grow winter squash varieties that would otherwise cost £6-10 each and they taste so good. I don't bother with butternut because it's so cheap and plentiful at the supermarket.
Courgettes give you a good return on investment - as do tomatoes (normally - last year was awful). Both and pretty easy and low maintenance. If grown in the ground which has been enriched with manure and good compost don't need any feeding. I don't feed my ground grown crops much if at all - and I grow lots of comfrey which can be spread around things to feed them for free.
I think if you're careful you can go it cheaply enough to either break even or be cheaper than buying stuff, but it's also very easy to spend loads.