Current-gen (LTO-9) gets 18TB uncompressed/45TB compressed, per
$100 tape. Upfront cost for a current-gen tape drive is expensive both
new and
used, but the media is designed for long-term, business-grade archival. You can expect tapes to last at least 2 decades, if not longer. Also does ECC. Transfer speeds are somewhat slow at <500MB/s, but this is for archival, not instant retrieval or running a server. Power consumption is somewhat low at
<40W continuous, meaning drives can easily run off of a UPS in no-power situations.
If drive cost is a concern, try going back a generation or two (check the Wikipedia article to find storage capacities). Used LTO-8 drives go for about
1/3 the price of LTO-9 drives. LTO-8 tapes sell for about
1/2 the price of LTO-9 tapes at 12TB uncompressed/30TB compressed.
As far as backwards compatibility goes, newer tape drives will read tapes from previous generations. LTO cartridge form factor hasn't changed since its inception in 2000.
To be clear, LTO's application lies in keeping an offline backup of all of your important files. I.e. it is for parts 3 and 2 of your
3-2-1 backup strategy.