Kathryn is 7 months, I don't remember how much we were spending at the newborn stage, but now we spend:
(per month in US dollars)
$200 on formula
$30-40 on diapers (as the diapers get bigger they fit less in the box so you need to buy more but they are at the same price as the small ones)
$8 on wipes
at least $30 on clothes per month (like you didn't realize you'd need more pants or shirts, etc. or you didn't realize they'd be this big this early so you don't have a sweater and it's still chilly out, etc. plus if I see something really great that's on sale or something)
under $20 on solid food, since the jars are so cheap, it's the baby cereal and baby yogurt thats more
we usually get her a new toy every month or so (get bored of the old simple stuff, and needs toys that are good for what she's going now-sitting, trying to crawl), so that can range from $20-$40.
Also don't forget about all the stuff you need to get before baby is born:
crib (mine was $250, it converts to big bed though), crib mattress, sheets and blankets, changing table, dresser, clothes, towels, bathing supplies, car seat, stroller. And more that I'm forgetting.
and the things as they get older:
toys, entertainment center (jumperoo or excersauser, usually $80 +), high chair, playpen, etc.
of course you get some things as gifts (I got high chair, stroller, car seat and playpen as gifts (totaling $650) and some clothes, blankets etc.
Yup, babies are very expensive!