TBH I've never really sat and worked it out but I do shop around for bargains and bulk buy stuff that's on offer. I breast feed so I don't need to buy formula or anything...although nappies work out quite a bit but with 10 people to do for something had to give.
Once a month or so I go out and spend about £250 on restocking the cupboards and freezer then I buy as and when we need things the rest of the time.....but saying that it's been a good few months since I did that and our cupboards are a bit bare (a la Mother Hubbard

).