A backup tape unit starts at about $1200 and up for a unit that will store 400 GB in native mode, or up to 800 GB in compressed mode. The tapes for such units cost about $25 each. And you may need to buy a SCSI card to connect to the drive. For higher-capacity tape units (say, 3 TB), add at least 50% more.
Compare to buying an external HDD unit of, say, 2TB that uses a USB3 connection for about $100. You could buy many of these and just load them up with files, then store on a shelf. And those files would NOT be compressed, so ANY system could read them back with no software help.
You cannot copy Hi8 tapes to a backup tape drive directly, although maybe you realize that already. The Hi8 tapes probably are analog video recordings, so you need a video capture card in your machine to convert them to digital format files that can be stored in a computer drive (HDD or tape). If the Hi8 tapes were originally recorded as digital video, then you'll need instead a way to transfer the digital files into your computer. For example, I have a Sony digital video camera that uses tape (could be Hi8 tape or digital tape) that I connect to my computer using a Firewire 400 (aka IEEE 1394a) connection for file transfer with appropriate video editing software. Once on the HDD, they can be copied to any storage medium.