For example, at Amazon in Europe an LTO-7 cartridge is EUR 48 for 6 real TB (EUR 8 per real TB), while an LTO-8 cartridge is EUR 84 for 12 real TB (EUR 7 per real TB).
At Amazon USA I see now $48 for LTO-7 and $96 for LTO-8, i.e. $8 per TB if you buy 1 tape. For larger quantities, there are discounts.
Older formats like LTO-6 have the advantage that you may find much cheaper tape drives, but you must handle more cartridges for a given data size.
Currently the cheapest HDDs are the external USB drives with 5400 rpm, which are much slower, about 3 times slower than the tapes, but even those are many times more expensive than the tapes (e.g. $27 ... $30 per TB).
That was a better than usual deal but $15/TB is the norm for low end drives with a bit of patience in the US, at least until Chia's influence hit extremely recently.
Sources:
https://geizhals.eu/?cat=zip&xf=1204_LTO-6 (tapes)
https://geizhals.eu/?cat=hde7s&sort=r (disks)
(For completeness, the tape library ran me 500€ on eBay used, but they normally run about twice that. It's a 16-slot library, which coincidentally matches the breakeven - filled for 40TB it's 820€, the same in disks would've been 800€, though I would never buy the cheapest crappy disks.)
FWIW, a major reason for going for tapes for me was that at some point my backup HDDs always ended up used for "real". The tape library frees me from trying to discipline myself to properly separate backup HDDs ;)