Forget landfill-bound party banners, balloons, napkins and other disposable goods. They're wasteful, chintzy and expensive. Instead, place a few single-stem bud vases on each table and fill them with pretty wildflowers. Add a few white votives, and you've got an intimate party atmosphere that cost you next to nothing.
You can also save money by investing in some basic linen tablecloths and napkins. They aren't too terribly expensive and can be reused later (like for a certain friend's future baby shower). Just be sure to go with ivory, as it matches everything and shows fewer stains than white.