Walmart offers the LEGO Super Heroes Knightcrawler Tunnel Attack for $38 with free shipping. (Amazon and Target charge the same with free shipping for Prime members.) That's the lowest price we could find by $4, although most charge closer to $49. This 622-piece set includes Batman, The Flash, and two Parademons minifigures.

Update: The price has dropped to $37 at Walmart and Amazon. Target charges pennies more.