We've shown you before how to make a softbox from a sheet of paper or construct one from an umbrella. For an option between those price extremes, try making one with a storage box from perennial Lifehacker favourite IKEA.
At IKEA Hacker, Thomas details how by simply cutting a flash-sized hole in the side of the Komplement box creates a handy softbox (used to diffuse light) for macro photography that also collapses for easy travel. (My own wardrobe is stuffed with that exact model of Komplement box, though it doesn't seem to be on the IKEA site at the moment.)
Camera flash soft box [IKEA Hacker]