When my sisters kids were newborns she went through 13 diapers per day. She spent around $100.00 per month on diapers although she didn't use them all in a month.
So probably more around $80.00 per month in disposable diapers.
When they got older she was only spending around $50.00 per month per child. As your baby gets older they will use and need less diapers so your cost will go down.
But in the beginning you'll be changing your newborn baby a lot.