The stuff we used on our walls at LinkUp.com is less than a $1/sqft and I can use permanent markers on it: http://www.solutionsmb.com/010~Graffiti_Solutions/035~MB4500...
It is made by a little company out of Canada that doesn't know marketing but sure knows chemistry.
I even uploaded a pic showing it over some of our wall art: https://plus.google.com/103755913372126150486/posts