I have prepared for this baby extremely inexpensively overall. Diapers, formula, and bottles have been my largest expense, and if you are exclusively bf-ing, as I am sure you will be unless you also have other circumstances, the bottle and formula thing are not even an issue.
One thing I have really realized with this is that for all of the people who choose not to bf for more than a few weeks or months, I think they must be off their rockers! That stuff is ridiculously expensive in addition to all the other reasons babies should be nursed if possible.
If I didn't HAVE to supplement, there is no way in the world I would CHOOSE to have to pay for any of that. Geez. Anyway.... My little rant of disbelief is over now

I had no idea just how much it was cause I exclusively nursed my first two and being on a limited budget and planning for a baby is made much harder with that additional expense.
Also, this time around I am not doing cloth till I get my own w/d. That's an additional expense I have
chosen though, cause I know my limitations at this point, and we're making it a more expensive choice to buy a more enviro friendly type, but cloth diapers are always of course the best choice.
And cloth are FUN to buy, and you can find all kinds of great stuff gently used at your local cosignment shops and every where else, like online. Be careful, cause buying cloth diapers can be addictive, so don't buy more than you actually need! Not that I know anything about that.

But those are the main things I have been stockpiling. Ordering cases of diapers from Amazon when they run a good sale, picking up a can of formula out of each grocery trip to stash, etc.
Also, all the clothes you will ever need for baby can be found SO inexpensively at thrift stores and garage sales. I think I have way more clothes than I will need for the first 6 months due to hitting thrift store half off baby clothes sale days twice by chance. Wow. SCORE! I was SUPER picky about what I bought or didn't. If it had the tiniest stain or wear I passed it over, and still ended up with a TON of clothes, blankets, towels, even a few slings, etc, all of it for no more than $30. All the way up to size 18 months or so. Not as many in the larger sizes, but if I saw something I really liked and it was less than a $1 I'd grab it to stash till it will fit.
Most of the onesies and footies I bought ran about $.50-.60 on those half off days. All pristine.
Clothes are easy to stash away, too.
I have so much that was given to me that I never would have bought on my own, but took cause it might come in handy. There is an abundance of free baby stuff. I was offered 3 cribs and 3 pack and plays, and probably a dozen bouncy seats

Seriously! So I took one of each. (I never have used a crib, but actually plan to at time this time). So when the time comes you'll probably be surprised at what you'll be able to get for no money.
And for the most part? You can certainly get by with a carseat, baby carrier (sling, meitai, whatever your preferences are), stroller (I am partial to using these, though not everyone is, and even if you wait a few months for one at that point a $10 one from Target will work just fine for occasional use!), diapers, some clothes (amount based on how often you are able to do laundry), and you're about all set for a good 6 months. Oh, and a bouncy seat! They are super convenient to set the baby in when you need to pee or whatever, and they are apparently EVERYWHERE for the taking for free. Seriously. At least a dozen offers! May as well. So, you could stash some of that, or wait to get it.
Of course, with what I previously mentioned, some things are dependent on circumstances, like for instance if you don't drive often but mostly walk, the investment in a good stroller may be an expense you have to consider, etc. Or, say, if you do want to use a crib sometimes, then you'll probably want to buy a new mattress for it even if you get the crib itself used or free (which is what we did). These larger purchases would wait, though, I'm sure, if you're just in the planning stages and won't want to store all of that.
If you're planning beyond that, well, more clothes/diapers in larger sizes?
Gosh, you know, sure they get more expensive as they get older, but now that I am thinking back on it, they sure don't cost a lot for a good long while.