Taking into account the rent for my double plot, petrol for the rotavator, seeds, fertiliser and the odd few pounds spent on seed trays etc. I spend no more than £200 per year on my allotment, which equates to the princely sum of £3.85 per week. For this I get fresh air, exercise, good company and more fruit and veg than we can possibly eat, grown with no artificial additives whatsoever. Everything is absolutely fresh and we can choose the varieties that we like. Surplus is either frozen, made into jams and chutneys or given away to neighbours. The sense of achievement when putting a meal on the table and being able to say "I grew all of that", (except for the meat, which I don't eat anyway) is second to none.
There is no doubt that we wouldn't eat as well as we do if I had to buy fruit and veg from the supermarket, nor would it be as fresh, plus none of it is plastic wrapped so it helps the environment to a degree.
I dread the day when I am no longer fit enough to cope with the plot, although I hope the exercise will keep me healthy enough for a good while yet.