I buy vegetable at wet market after work for next day's dinner. And I buy meat etc at supermarket once/twice per week. I ask my 工人 to supermarket only when something is out of stock.
I carry shopping bag with me every working day. I feel I am a 工人!
Like your maid, my maid loves to walk to Mongkok market too. The reason is the same as yours.
If 3 dishes for dinner without boiling soup, average $60 should be enough. If we want to boill soup,we need to spend about $100 for that day. However, we usually buy some more food for breakfast and lunch.
買餸$400一個星期, 煮共六餐晚餐 (2公婆 + 1工人 + 1個小朋友), everyday煲湯,2-3碟餸. Already enough la. (sometime she needs to cook lunch for us also, like this few days are my daughter's exam, i take my leave to study with her and we eat lunch at home)