Assuming you are referring to a plastic fog light cover, the cost will vary depending on the make and model of your vehicle. For example, a Honda Accord from 2008-2012 costs about $15-$20 for just the cover itself. If you need to replace the entire assembly (cover and housing), then the cost will be closer to $100. The price will also go up if you have a more luxury vehicle like a BMW or Mercedes. You can usually find replacement covers at your local auto parts store or online retailers such as Amazon or eBay. If you need help installing it, most auto shops will charge an hourly rate plus the cost of any parts needed.