How much did it cost in 1980 to buy a tape drive capable of backing up 5 megabytes of data? Both for the drive itself and per tape?
The biggest issue here might be that in 1980 most users didn't have hard drives. Mind you, Tandy was at that time selling the Model II withoug hard disk, but 1-4 floppies as professional system. Even the lowest cost hard disk setup in 1980 was still way past 3000 USD, with the drive alone being way past 2000 USD - a well fited S100 system including a floppy would cost less.
A Corvus S100 HD subsystem would be close to 5000 USD - which isinteresting here, as Corvus already offered a tape backup solution in1980, the Mirror Backup System, which could use a standard VCR to backup the harddrive. Several full backups could be made onto a single standard tape. This added another 800 USD for the option plus ~1000 USD for a VCR. Compared with a professional QIC backup system a real bargain.
So if one had already bought a Corvus disk system, adding backup would be somewhat less than 2,000 USD. Tape cost was that of a simple VCR cassette, less than 10 USD.