Walmart offers the K'Nex Son of Serpent Coaster Building Set for $505.88. Opt for in-store pickup to knock it to $493.23. That's $13 under our mention from four days ago and the lowest price we've seen. (It's the best deal now by $110.) This 5,000-piece kit stands over 5 feet tall and 7 feet long when assembled. Also included are speakers for an MP3 player, 16 K'NexMen figures, five coaster cars, and three motorized chain lifts.