for replacement cost you should contact their service center(if under warranty), otherwise search in local market. Cost in local market will be three-fourth to half and you also save labour cost.. it should come around 3.5-4 or 5K for just the burner, I think it should come to 3.5K only, but anyhow it is not 7K. and external burner from LG costs around 3.5-3.8K the slim ones and same from sony they cost 4.5K
What is the problem with your burner. check which device driver for the model.
If your part is not inwarranty take it out yourself (check in the bottomside of laptop there will be a screw for burner unscrew it and take the burner out. and get a similar burner for yourself. most of the burners supplied in laptops are LG burners.
@izzikio_rage, there is something called warranty, the service center will replace it as many times if it defective.