Walmart offers the Bestway H2O Go Single Lane Water Slide, model no. 52198E, for $7.43. Opt for in-store pickup to avoid the $4.97 shipping charge. That's the lowest price we could find by $9, except for the mentions below. It measures 18 feet long.
A close price: Both Target and Toys"R"Us have it for $7.99 with in-store pickup.