I was thinking about this this afternoon. I grow quite a few veg, but mostly not in huge quantities and although it's lovely to have fresh veg when I do I don't think most of them save me that much money. I'd be really interested to know what crops people think save them money and which they just like to grow as they taste better and which just don't seem to work very well.
My thoughts are below if you are interested(warning very long):
Tomatoes. These have just started cropping this week and will be over by the end of September. Say 9 or 10 weeks season max, 2 weeks of which I'll be away so the neighbours will benefit rather than me. Now I usually spend about £1 a week on toms, so will save about £8. But the seeds cost £2.50 and the growbags £2.50, So although I'm getting more and tastier toms, I'm only saving £3 and that's ignoring cost of any pots/compost used to start them off and cost of water (metered, though mostly comes from the butt) to water them.
Peppers - just one so far from 3 plants! Not really worth it.
Melons - 1 so far from 4 plants and looks like 1 more on the way. Seeds expensive and used 2 growbags - not worth it (but the first was delicious and I've never eaten a melon within 3 mins of picking it before)
butternut squash plants (2) last year produced 2 avocado sized fruit between them last year and it looks as though I'll only get one (hopefully slightly bigger one) this year. It's the same seed packet, but I still don't think I've recouped the cost of the packet yet.
This year I tried a new (much more expensive) squash variety: Crown Prince. Cost about £5 for 5 seeds (I know, I'm mad). I was supposed to get 4-5 large squashes with good storage ability from each plant, but one didn't germinate and the others look as though they'll only produce one fruit each (if that as they look like the plants are getting mildew). It remains to be seen how large they'll be or indeed if they'll rot away, but even 4 fruits for £5 doesn't seem a huge bargain.
Lettuce does seem like a good deal, I've had a good supply and not had to buy one since May and the seeds aren't expensive either. Must have saved a fair bit there.
Carrots are going well too, but I didn't grow all that many (probably a couple of months worth) and I usually only buy fairly cheap ones in any case so saving isn't huge, but worth growing for taste alone.
Courgettes: I use a couple a week and the plants have provided that (Not much more), but only over a fairly short time. They seem to have powdery mildew now, so if the dilute milk I sprayed them with today doesn't fix them the season may be over for this year). Probably just about worthwhile though - actually definitely worthwhile as they were last years seeds.
Onions seem good - I buy sets for about £3 and usually produce about 3 months supply which seems good value (though a bag of value onions is very cheap for loads).
I've never had much success with brassicas or sweetcorn on either quantity or quality, so lose money on cabbage, brocolli and cauli (get one or two poor specimens of each if I'm lucky a year), but I'm trying PSB this time so perhaps that will be better. I always love the idea of homegrown sweetcorn, but it never usually germinates properly.
Rhubarb is excellent value - more and more comes up year after year.
I've grown swedes, parsnips, celeriac and leeks this year for the first time, but it's too early to say if these are a success or not.