Hi and welcome to PF
As FP said, it completely depends. When I was pregnant with our son after our first scan each week we started buying a packet of nappies and wipes into our weekly shop. We barely noticed the price difference each week and by the time he was here we were stocked up for ages
We also bought neutral things (unless you know the sex) whenever we had spare money and saved up so when he was born and we knew the sex we went on a shopping spree. One of my favourite days! I also breastfed so didn't worry about formula. Its one of the main things you can't stock up on because you don't know what milk will agree with your baby so best to wait. Again just add that into your weekly shop (about 10 pound). When babies start to eat if you wait till 6 months to wean they can eat pretty much whatever you do so you'll barely see a price difference in that too!
If it's your first baby you'll probably find people will be keen to buy you things. With our first my parents bought us the nursery furniture, hubbys parents bought us the pram and my sister bought us the moses basket. We're obviously trying for our second this time so we're budgeting to buy everything ourselves. We've put away about 1200, we've worked out this will buy us our pram, moses basket/stand, cot and set of drawers (got a big cupboard in the room so don't need a wardrobe). Just look up the prices of things to give you an idea. Like everything you can get a pram that's £200 right up to 1200, moses basket for £20 or 100. Just have a look at what you like and save it, that's what I do. Or there's always second hand depending on your budget!
For us we also have to factor in child care, so this is another thing we save for. If that applies to you then try to work out what you'll be and put as much away as you can afford. It'll make such a difference when you've potentially saved enough childcare for your costs to be halved in the first year
Also, if you work calculate what your maternity pay will be so you can also budget for this.... or you don't need to be as anal as me, you could just throw it all to hell and get pregnant
Hope this helps a little xx