You can find out over at The Motley Fool using their calculator. You enter things like daycare, groceries, bigger home or car, more insurance, and dental bills along with the child's age at the end of those expenses, and they total it up.
It seems to us it'd be pretty hard to estimate those expenses for a child's life, especially if you're a first time parent, but it does open your eyes to some of the costs you might not have even thought about.
Find the calculator here.
Image via Stuff.co.nz.