Lets think... Yesterday I picked up a sarnie, a drink, and some crisps. Think I spent about three quid.
I then cycled out across the fen, picked half a kilo of wild mushrooms and a bag load of wild salad.
Should I have to pay market value for a whole load of wild herbs and mushrooms, I'd be in profit by about £30. Obviously, I wouldn't be buying such things
I don't worry about spending two, three, five or even seven or eight quid on lunch. Most of the food we eat is from the allotment, bought in bulk from local wholesalers and farmers (especially meat), or picked wild. We both cycle, saving dosh there too of course.
Is there any such thing as a free lunch? Probably not, but learn to identify a few wild plants to be found in nearly any urban park or wild space and you can effectively run a healthy profit.