Tanga offers the 6-Foot Whiteboard Wall Decal for $6.99 with free shipping. That's a buck under our August mention and the lowest total price we've seen. (It's also $1 below the best price for a similar decal elsewhere.) It sticks to most walls and peels off without any residue.
Looking for a chalkboard version? Walmart offers the Con-Tact Self-Adhesive Chalkboard Contact Paper 18" x 6-foot roll for $5.79 via in-store pickup. That's $2 under our August mention, a current low by $1, and the best price we've seen.