I can't really recall seeing any website for a company that converts media files from one format to another spelling out exactly what it costs per cassette tape. Perhaps I've seen it before and just don't recall. That said, I'm treating http://mediarenewer.com/Services1.html as the first site.
I have ~1000 cassettes in ~20 boxes in the sump pump storage room in the basement. Given the following price of $6.95 / CD, as shown below, it would cost (1000 x 6.95 =) $6950 to pay that company to convert my cassettes.