Amazon offers Prime members the American Crafts Erasable Chalk Markers 2-Pack for $5.25 with free shipping. (Walmart charges the same via in-store pickup.) That's the lowest price we could find by $6. They work on a variety of surfaces including chalkboards, dry erase boards, windows, glass, mirrors, metal, ceramic and more.

Update: The price has increased at Amazon; however, Walmart still offers it for $5.25 with in-store pickup.