Its called a decorative skid plate. BMW part number is 82110021339. Price in the catalog is $209.00. It is not metal but is made of urethane so you will need to factor in painting as well. I hear the install is very easy.

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Current cars:
03 BMW X5 4.4i NAVI (Dinan software upgrades-tranny and Engine ECU, 4.6 body kit w/ 20's staggered wheels, aluminum running boards, M3 SMG steering wheel, eisenhaus race exhaust, predator angel eyes, 6000k HID fogs, hardwired V1 radar)
06 Honda Odyssey EX-L RES NAVI (4300k HID's)
00 Mercedes S430 SPORT NAVI
08 Honda Accord V6 NAVI sedan (4300k HID's)