Get a 4x8 sheet of shower board from Home Depot. It's basically just a thin piece of melamine, and should be like 8 bucks.
Get a 4x8 sheet of really low grade 3/8" or 1/2" cedar chipboard (like that used in flooring), again maybe 3-4 bucks.
Glue the former onto the latter with Liquid Nails, probably 2-3 bucks a tube.
Works great, erases, and when it finally gets too gross to use (in about a year), you replace it for another fifteen bucks.