Some disks do deteriorate over time. Some grow mold (really!) and some are simply cursed (i.e. the infamous Iomega Zip disk click-of-death). If we can't read anything at all, and we securely dispose of your media here - you will owe nothing. If you do want your media back, you'll pay the return shipping charge you were originally quoted. If you didn't consider non-readability when you asked for your quote, you can plan on approximately $5 over a standard USPS postal rate based on weight and distance.
If you fall somewhere in the middle - that is, if some disks are readable and some are not - then you'll pay the rate your were quoted (even if it was discounted based on volume) for the disks that were successful, down to a minimum charge of two if there's at least one viable disk.