CG, I'm echoing Velo on this one. Costco ftw.
Plus, you're going to be needing a pallet of diapers pretty soon anyway.
I never had any plans to ever get a membership but when we bought our house some good friends of ours bought us a membership as a housewarming/Christmas gift. We used it so much we not only renewed but upgraded the membership because we were spending enough to get the upgraded membership for free and then some.
EDIT: Seriously, you can save the cost of membership on either diapers or formula. We actually did figure out we saved ours on formula alone after Mrs. Velo stopped breast feeding one year.