The cover of one of those vinyl, 3-ring notebooks makes a great homemade Dry Erase Board. Use the kind that has a pouch (and is often in back cover of the notebook, too) that gives space for slipping a cover sheet into. The cover sheet can be a page of math facts, a fill-in-the blanks page, a dot to dot page ...or simply a blank page for students to draw and write upon. Works really well!
Experiment with different markers. The one pictured is a washable one by Crayola. In other words, it doesn't always have to be the official dry-erase ones. Test any you are not sure of in an inconspicuous place first.