In the beginning, not very much, and you'll get a lot of stuff as gifts. Biggest expense is probably diapers.
But don't get lulled into a false sense of security.
Education, including the extras like music lessons or sports or religious training or tutors. Think about what just $50/wk comes out to over 10 years, if you're lucky to get by with that little.
Summer camp.
Extra person(s) on vacation (we don't even take vacations anymore), the choices of which will be much more limited.
Health insurance (BIG TIME if you, like I, don't have a good deal with your employer)
Responsible parents should have a pretty big life insurance policy.
Maybe you'd otherwise live in a smaller house.
Want to go to a movie with your wife? Add $20-30 bucks for babysitting on to the ridiculous ticket prices, plus the hassel of having to set everything up several days in advance. Will you still go to the movies at $50 a pop?
And with #4 due next month, I've got to buy a mini-van or something when my family car is paid for and in good shape.
Luckily, it's only money.