Amazon offers Prime members the Darice Jumbo Sidewalk Chalk 20-Count Bucket for $4.27 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $7.44.) That's the lowest total price we could find by $4. This chalk is non-toxic and washes off with water.

Note that this item currently has an expected one to two month shipping delay.