I think it depends what purpose/rights they have purchased with their disc already. For example; when I sell a disc of image to a client, I'm not giving them unlimited usage rights, I'm giving them the right to make prints or copies for personal use. So they would be allowed to make their own copies of the disc if they wanted. If they came to me for another disc, I'd charge a nominal fee for my time to make the disc and material etc...Maybe $20.
However, if I had sold them a disc that was strictly for viewing (sideshow) and did not include print/copy rights...then the price for an additional disc would probably be the same price as the initial disc.