What he said!Have they already paid digitizing fees for the designs? If so then in my opinion they are entitled to the files. To charge any additional fees to "release" the files to the client probably won't sit well with the customer and I wouldn't blame them. It would look like you are holding the files hostage.
If they haven't paid for the digitizing of the files then you could advise them normally there is a digitizing fee for each design. Let them know the files were digitized at no charge for use by your company but can be given to them if they are willing to pay the digitizing fees.