Im a nanny and normally spend less than £50ish (more often than not MUCH nearer £40 a week) on pretty much all organic food too.
It can be done (tho i must stress that is for me and 2 children only and only for 5 days a week, tho the fruit and veg i buy normally last all week and the parents use them as well.
Agree cooking from scratch helps!
I go to tesco alot where organic butter is only 88p for a normal size one etc (lurpac is around £1!) and their own brand organic food are very good value!
I will ONLY feed them organic meat (esp chicken etc) organic fruit and veg, and make an effort to buy lots of other 'stapels' flour / eggs / pasta / tinned toms / tom puree / cheese / yoghurt / milk / ham organic as well.
The children eat 'well' at meal times and have 1 fruit snack in the afternoon (or an organic cheese stick or home made cake / biscuit etc!)
When you cut out all the 'junk' multipacks of crisps / choc bars / cake bars / biscs etc its amazing how much the shopping bill is reduced buy.
We also make our own ice cream etc,a nd lots of our own puddings. They only drink water and apple juice is an occasional treat!
I find that its processed ready made food that costs more than organic, and i know what i would rather eat and feed the children!