Dick's Sporting Goods offers the Shock Doctor Adult Gel Nano Convertible Slim Fit Mouthguard in several colors (Black pictured) for $12.97. Opt for in-store pickup to dodge the $5.99 shipping charge. (Walmart offers it for cents more in Pearl Carbon.) That's the lowest price we could find by $7. It features upper/lower molar cushions and a quick-release tether.

Update: Walmart now offers it in Pearl Carbon for $11.82 with in-store pickup.