Drive enclosures, raid/etc interfaces, and motherboards burning electricity make it a lot more complex than raw HD's vs raw tape. Tape libraries cost a fortune, but so do 10+ racks of cases+power supplies+servers needed to maintain the disks of equal capacity.
Tape suffers from "enterprise" which means the major vendors price it so that its just a bit cheaper than disk, and they lower their prices to keep that equation balanced because fundamentally coated mylar/etc wrapped around a spindle in an injection molded case is super cheap.
Tape suffers from "enterprise" which means the major vendors price it so that its just a bit cheaper than disk, and they lower their prices to keep that equation balanced because fundamentally coated mylar/etc wrapped around a spindle in an injection molded case is super cheap.