The price varies by model & region, but it usually is not cheap. Take the laptop to Apple or an Apple Authorized Service Provider for an estimate.
Personally if it is just cosmetic, then I wouldn't bother replacing it. If the dents affect opening or closing the display, then that is a different matter although I have been successful in carefully adjusting the corners. However, extreme care must be used or the LCD and glass could break. This procedure could also add some more marks to the corner. It takes practice. Most techs probably would never attempt it.