How could one make a DIY Gray card for exposure/color correction?

I know that there are "gray cards" sold that help with exposure/color correction, but as I'm on a tight budget I'd like to make my own. Is this possible?

Also, are there other objects (paint sample cards for instance) that can be used for color correction?

Answer

Gray cards are super cheap (e.g., $2.49) - I wouldn't bother. :)