I purchased some of these, which you get at about $0.16/sqft:
Unfortunately they also suffer from the problems with not erasing things after a while, although you can still clean them with water after that and it works. Also, they're very thin and tear easily.
EDIT: Also, this thread is making me think that perhaps searching for "clear plastic" on Amazon and buying one of the things that come up to put it on a white surface could also work. Like this. People replace glass with this material to make greenhouses; maybe it would be a good glass replacement for whiteboards, too...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.