Allotments v Farms

I would have thought that 1kg/sqm would be the minimum to expect unless the soil fertility is very poor, when your aim should be to improve it quickly.
Looking at Hessayon (the vegetable 'expert') expected yields (all in imperial):

beetroot 1lb/ft is 3lb per yard, say 5kg/sqm.
kale 2lb per plant, 18" between say 4 plants/sqm 3.6kg.
onions 0.8llb/ft row: 3' x 4rows gives .8x12x0.454=4.3kg.
a good cabbage could weigh a kilo and you would want more than one a sqm. A brussels sprout plant should give a kilo too and a broccoli plant 0.75kg.

Tomatoes and cucumbers in a greenhouse would be lot as tmtt says above.
The problem is, you may grow a good crop but there are a lot of little beasties that will be wanting to help eat it all.
If you are looking at 80g portions, some would be fine straight from the garden: apples, raspberries.. but others need trimming/peeling parsnips ...