400 pln works
I agree with that as a base figure. It's possible to spend less (many have to) and it's possible to spend much more. It all depends on how many basics you buy and how many extras.
I would guess that our two person household spends around 400zl max. per week. For that price we don't buy much processed food (we make our own bread and cheese, don't buy cakes, biscuits or much ready made) but we do buy good coffee beans and some alcohol which drives the bill up. We also eat out sometimes which keeps the bill lower.
If you entertain guests, the costs will be higher.