Walmart offers the K'NEX Firestorm Freefall Coaster, model no. 51539, for $19.97. Opt for in-store pickup to dodge the $4.97 shipping fee. That's $2 under our expired mention of a shipped unit from two days ago and the lowest price we've seen. (It's a current low by $33.) This 674-piece set features a motorized chain lift, 18-foot track, and it requires two AA batteries (not included).