The cover is made of thin metal, cost around US$180 list but can get it for as low as US$130. Give the part # listed by MrOtocinclus to a local dealer to find out their price.
The cover serve as part of underbody aerodynamic setup, smooth bottom equal less turbulence and lift. It also helps keep small rocks and other debris from hitting up the engine bay.
Mine was slightly damaged by snow, but the dealer tech managed to make a temporary fix for me. It cost around $300 here in Canada, will buy one in the US next time I come down for a visit.