White Foam Board by Elmer's
Cheap, accessible, and functional. If that's not the trifecta of value, I don't know what is. These white foam boards are an essential item for any home studio's lighting setup. I personally have two by my side at all times.
Position them opposite a light source (for example, a soft box or a window) to help softly bounce the light and even out the shadows on your subject. Whatever brand you go with, make sure the surface is matte and thick enough to stand sturdily without folding over.