I could never feed us (3 adults) from the garden, even though we've got a large one. Instead, I supplement our diet with homegrown stuff - currently growing cress on the windowsill to go into egg mayo sandwiches - I try to keep some on the go all the time, which saves me about 50c a week (which is what it would cost to buy from Tesco, if Tesco actually had it in stock). Similarly, I grow chillies on the windowsill - usually end up with enough for a few months, saving approximately 99p a week on the Tesco version.
Right now I'm growing Little Gem lettuce - again, a saving - but not a hugely significant one. I'm looking forward to foraging for wild garlic, but it's not going to fill us up.
I spent €22 on a pack of 22 thousand seeds - various vegetables, herbs and a few flowers - it might end up saving us about €50 over the season, so it's not much, really - and if my courgettes are as successful as they have been before, my DH may threaten to leave home before he eats another one - you can have too much of a good thing.
One activity that's actually worth the bother is making wine from growing things - parsnip, rosehip, elderflower, gorse, blackcurrant, blackberry, elderberry - even a version of sherry from a parsnip and sultana mixture - all good fun and definitely a saving on shop-bought booze. All you need to buy is yeast and some sugar (or honey for mead, which is also worth making) - most of the rest can be grown or foraged. The results can be...erm...varied - but I don't often make something that's completely unpalatable.
Living on the coast, we could forage for mussels, winkles, sea urchins and razor clams - but we rarely do because they're labour-intensive and DH isn't a fan of seafood. His attempts at fishing got us one nice seabass, once, about 10 years ago - the rest was dogfish, coalfish and - if we were lucky - mackerel. You could never rely on it though, the weather and the vagaries of fishing being what they are.
I would never forage for fungi without a fungi expert, but they do grow round here, though not in 'let's have mushrooms for dinner' quantities.
So yeah, growing food and foraging are all great as hobbies, but not something you could replace your weekly grocery shop with, in my experience!