Sheet plastic, styrene, resin, sintra, cardboard, fiberglass, foamies, balsa wood, wood, the possibilities are endless and only limited to what is available to you. For durability and cheapness, cardboard sandwhiched between some plastic is REALLY cheap and durable, and cake to repair. Or just layer some styrene together and glue together for a sturdy blade.
Aluminum I wouldn't reccomend unless you will never be wearing them out in public. You don't want something that could actually hurt someone, and given the diminished visibility when all suited up, it's alot easier to smack into someone with them and cause harm. Plus, I can't think of anyplace that would allow for aluminum/metal blades.