I wonder what kind of tape they are using
Over the last 2 decades every tape I've seen have been about as and often even more expensive than disk drives at the same capacity. It seems unlikely to me that a fairly complex thing like a tape changer can be cheaper than the electronic overhead of a disk array. After all that's as cheap as disk, and if you are contempt with tape-like performance, you can even heavily overbook the bandwidth to the drives.