GameStop offers the Hasbro Star Wars Rogue One: Rebel U-Wing Fighter for $15.97. Choose in-store pickupto avoid the $5.99 shipping fee. That's the lowest price we could find by $21. It fires NERF darts (two included) and includes a Cassian Andor figure.
Update: It's now available for in-store pickup only.
Hasbro via eBay the Nerf N-Strike Thunderblast Launcher for $12.49 with free shipping. That's the lowest price we could find by a buck; most stores charge over $20. It includes two missiles that fire up to 60 feet.