The price can vary a great deal depending on the condition of the audio tapes and number of separate tracks etc. To have full navigation on the CD, the audio would need to be split into individual tracks, otherwise it would be one long track on the CD. Bear in mind that a CD will hold around 70 minutes of audio, so we might need to use two CDs per C90 tape. I would need to check them out before giving a fixed price but as a very approximate guide, whilst there are so many, it could be anywhere between £20 and £30 per cassette.