It depends on where you live! I am in the US and I bought a lot of clothes from Babies R Us. I am a clearance rack shopper. I was able to sift through about ten of their clearance racks (and if you're familiar they are big circular racks with two levels each) and found a TON of items for 2 dollars or less. I also got a ton of onesies there when they were having a 50% off sale of all their brand onesies and ended up finding clearanced ones there too that could take 50% off of as well. I got lucky.
I bought my jackets from The Gap when they have their 40% off sale (that happens once a month or so for a day two). I got khaki pants for under 3 dollars a pair at The Children's Place on their clearance rack. Paying such cheap prices for a lot of items, I was able to spend a little bit more money on a few things like the jackets, a few overalls (still half off), and I bought a ton of sleep and plays for regular price (or just slightly off).
I bought all my sleep bags through kids.woot.com when they have their Woot Off days. They are cheaper, but you have to be vigilant and stay online all day to get these deals when they pop up. I got several items from Zulily.com as well. Often they'll have a brand pop up and sell 7 piece sets for 7.99-10.99 and those are really good deals and the items are really nice when you get them.
I check both woot and zulily every day just in case there is a super deal going on (and that;s the only way you can really find out if it is a woot off day).
I also utilized babycheapskate.com they give daily updates where to find good deals, not just on clothes.
Sorry if I went off on a rant. I am quite happy with how much I've spent and what I've gotten. over 250 items for under 350 dollars and a couple were the more expensive items. So essential less than 2 dollars a piece and all name brand (except for babies r us brand, but they are nice too).
Now a break down-
In 0-3 I've got
20 short sleeve onsies
10 long sleeve onesies
5 sleep and plays
10 socks
10 pants
3 pairs of mitts
2 jackets
2 shoes
in 3-6 months I have
10 short sleeve onesies
10 long sleeve
15 sleep and plays
10 pairs of pants
2 jackets
10 socks
2 shoes
6-9 months
Several onesies (I'm not exactly sure on the number)
6 separate outfits
10 pairs of pants
3 jackets
3 shoes
2 overalls
10 socks
and my 9-12 mirrors my 6-9 except that I have a few more outfits here. I also have 3 overalls.
I like to have extras of things and I plan to do laundry once a week. I may even have more than that, I stopped counting at some point. Those are just my initial counts. I have clothes that go up to 2T, but have everything I think I'll need for the first 12 months. I hope I helped at least a little.
It should also be noted that I live in Sunny California, so I won't need snowsuits or a lot of other weather type of clothing. Just jackets and some warmer clothing for nighttime or if it just happens to randomly rain. And perhaps a raincoat when he gets a bit older.
If you're in the UK, my post means nothing! Good luck!