You could go as Pocahauntas, I created this quite cheaply. I went to a local material shop and bought some brown material, which I made into some sort of short dress, and plaited the left over as a belt (which held the dress together), I also used the material to cover some shoes in a mochasin like thing, and bought a indian wig for about �10 in a joke/costume shop. I think it all came to about �15 and was great fun to make.
You could do a number of things with cardboard boxes, too, with a bit of paint. like a dice, or a robot etc etc.