Congrats on the new soon to arrive baby. I hope everything goes well and he or she is born a happy healthy baby and you do alright through child birth. :)
For my babies diapers I spend $100.00 per month on disposables and I sometimes have a few diapers left at the end of the month.
I use Pampers diapers on my 1 month old. I read that babies start to use less diapers the older they get so your and my diaper costs could go down as the baby gets older and uses less diapers.
But in the beginning I'd budget at least $100.00 per month for diapers. Of course my baby might be using more diapers per day then some babies since I change my baby as often as possible as I don't like leaving him in a dirty or wet diaper too long.