At our old apartment, we just got a piece of scrap glass for a pittance at the local glass shop. Glass is awesome, it never wears out and is always easy to erase.

In a pinch, you can always write on the windows, though you can't read it if it's dark outside. ;-)

It looks like the Glass Whiteboard people at the link above coat the back of their glass with white material. But what you are saying appeals more to me because I'm on a budget, and you can just mount the glass to a wall that's been painted white.