I purchased some of these, which you get at about $0.16/sqft:

http://www.amazon.com/National-Brand-Write-On-Static/dp/B0000E2RGH/ref=sr_1_8?ie=UTF8&qid=1374840990&sr=8-8&keywords=white+board+paper

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...