I thought everyone knew $10 4x8' sheets of shower board from home depot was the shoestring budget whiteboard solution. $250 framed whiteboards are for the "Enterprise".
The best way to clean them I've found is 90% isopropyl alcohol. The bad side effect is your office will smell like a doctor's office for a while.
I got a big sheet of glass and spray-painted the back of it white with epoxy paint. Total cost ~$180 for a 28x68" whiteboard, and it looks amazing!
As a quick tip, if you're looking for big, inexpensive sheets of glass talk to your local glazier and ask if they have tempered glass mis-cuts. Lots of people order tempered glass in the wrong size, and seeing as they can't re-cut it often you'll pick it up for cost...