The cost of diapers per month and per year depends on the brand of diapers and price of diapers as well as if they have more than one kid in diapers or if they only have one child in diapers.
For one kid in diapers the parent may spend between $40.00 to $80.00 per month on diapers especially when the child is first born.
Newborns and babies under a year old can sometimes go through as many as 10 to 12 diapers per day but as they get older they tend to use less and less diapers.
When my kids were 2 years old they only needed 4 diaper changes per day because they wet less and the diapers I bought them which were Luvs diapers absorbed a lot so they didn't need as many diaper changes.
Sometimes when my kids got to 3 and almost 4 years old they only needed 3 diaper changes on some days because the diapers absorbed so much that they never felt wet.
I would change their diaper in the morning when they woke up at around 8:00 AM and then around 12:00 PM I would change the diaper again and then around 4:00 PM or 5:00 PM they would need another diaper change and then sometimes they wouldn't need another diaper change until morning again.
So as they got older I spent only around $50.00 per month for diapers which was not all that expensive.