Whiteboards are expensive. I bought a barely-used, magnetic whiteboard for $100, which I love.
I seriously considered this hack, but the whiteboard paint got poor reviews. Jessica pointed me to this whiteboard paint, which sounds better. I also considered "Bath Board," but it was too much trouble, and I wanted magnetic. Recently read about Go Write Dry Erase paper. Mixed reviews, but seems like a good option for portable, even temporary whiteboard surfaces. Another option is lexan sheets.
I ordered DrawTop to convert my MacBook Pro into a portable whiteboard. I was very unhappy with the product. It came folded and damaged, and it was too cheap for me to want to put on my laptop. I ended up cutting the sheets and putting them on clipboards.