MSN Money has a table showing the cost of raising a child to the age of 18, based on a survey by the US government from 1990-92 (in 2001 dollars):
The cost (red box is my edit) of $249,180 didn't even include the cost of sending them to college, or approximately 1/5th of gross income. If you have two kids, like me, that's roughly half a million dollars! (obviously not counting that elusive multiple-kids discount that other parents have been telling me about)
If you look closer at the numbers, a number of things are waaaay off base now, like the cost of healthcare and childcare. I wonder if they've done a more recent study, but I think I already know the answer.