Leyla is formula fed and that costs us about £14 a week, not including buying bottles, new teats etc, so it's not cheap!
I have spent a lot on cloth nappies but if I was only doing it for environment, cost and baby-bum-health reasons, I would have spent around £150 on a full stash of day nappies (you can get plenty of Flips or Econobums for that) plus a couple of night nappies so maybe £175 in total. Unfortunately, I love pretty nappies so it has cost me more than that! You spend less on nappy creams when using cloth nappies too and I've only ever had to throw clothes away due to horrible nappy explosions when using disposables. If we were using disposables, we would use eco nappies at about 15p each. We would use about 7 nappies a day so that would be £7.35 a week. Depending on what brand we used, disposables could be as little as £4.41 a week for Tesco cheapies, or £9.80 for Huggies. It would have been more when LO was younger. It's worth noting that I would change a disposable nappy more often than a cloth nappy because LO gets bad nappy rash in them.
For clothes, you could easily clothe your baby cheaply but I think most mums would admit to spending more on baby clothes than their own clothes! If you wanted to keep costs down, you could buy simple supermarket vests and sleepsuits, plus a couple of trousers, jackets/cardigans, hats and socks. Even when baby is growing fast, I think you could do that for £20 a month.
Toys can cost a lot too, but babies are just as happy to play with household stuff like some rice in a tupperware box with the lid stuck down so you don't need to spend anything on toys.
and he will be weaning soon (he already has carrot sticks and celery and the like for tastes while we eat) and because we are doing BLW he wont cost any extra because he will eat what we eat! So the only things i buy for LO are completely unnecessary things that i fancy buying like cute new nappies or cute clothes i see that he doesnt need or toys that he doesnt need! 
but because we use reuseables and breastfeed, there is never any time I NEED to spend money. So if we are skint, LO never goes without or we dont go without anything to fund LOs things iykwim. The only necessary expense i suppose is the initial nappy stash costs (but that is all included in your initial baby costs if you buy while pregnant, like prams and things) and clothes but as you say, they can be super cheap. I only ever shop at asda and tesco and cheap places, i leave the next and debanhams things to SIL for gifts! 
but I havent needed to really, I got addicted to pretties 
)
