I think the amount you need to make to 'afford' to have a child is so small, that it's not really worth talking about for the average American family (NB: in this case I'm talking about a married couple).

If you want to drive 2 new cars, put your kids into a private school and have a 5,000 square foot home, yeah, you'll need a fairly large chunk of change. But none of those things are necessary to be a good parent.