Kind of like asking the cost of a car + groceries.
You can pick up an old tape drive, like an LTO5, for about $1,000. You can LTO-5 tapes for about $15 each, with a capacity of 1.5 TB.
This setup puts the cost at around $10/TB, compared to somewhere around $30/TB for disk. Since you save $20/TB by using tape, it takes you 50TB before the total setup is less expensive than disk. Plug in your own numbers if you like.
You'll want to get a fiber channel card too, but that's easy enough.
Newer generations of LTO have marginally cheaper media (per TB) when you're buying in small quantities, but the drives are more expensive, and it's not uncommon to see poor capacity utilization in tape systems. The big players with lots of data will get newer LTO generations because they can save on library space, vault space, and drive time.
The advantage of tape is that you can easily and cheaply keep a daily rotation as well, slot in a monthly, and then an annual to really keep a good cold storage backup. I had to restore from a previous night's tape often enough that for anything critical I'd be paranoid about adequate backup protocols. I don't know the failure rate for tapes, but I think they're lower than mechanical HDD, and definitely last longer unpowered.
If you're in LTO5 land, you can get away with U320 SCSI instead of FC, and this can cut your costs in half. (Chances are if you're asking this question, you're the kind of user who won't be impacted much by the slower R/W speeds).
Also, library units are often cheaper than standalone drives for reasons I don't quite understand. I purchased a 12 tape autostacker for around $250 shipped, paid another company about $50 for the upgrade to make it 24 tapes, and another $50 for the HBA.
As a hobbyist with no commercial reason that could justify faffing about with imported tapes, I have a deep hatred for import taxes and the (occasionally very) steep shipping prices across the Atlantic. If I'm gonna have to pony up 200% of the original price to get the tapes delivered to my doorstep, tape drives might as well be fictional to me.
Even in UK, you can buy them from Amazon Germany or from Amazon France, at only EUR 7 per TB for LTO-8 or around EUR 8 per TB for the older standards.
Previously there were no customs taxes but after Brexit I assume that there might be some taxes, but they should not be so high as to increase much this low price.
However, if you buy external USB hard drives, it's only about $20/TB, you have full random access, you don't need a fiber channel card, and if you buy a bunch of USB hubs, you don't have to change tapes every 1.5 TB.
IMO the niche where tape still has an advantage is pretty small these days.
I can't speak to the old generations of tape, prior to 2000. The LTO cartridges seem fairly sturdy to me, and I haven't run into many problems with the media itself. Unlike hard drives, tape media is fairly insensitive to shock, which makes it easy to transport. I've seen cases of water-damaged media, cases where the leader pin broke off (and got taped back on, and the tape was successfully read), cases where tapes were physically lost, cases where the database recording the locations of tapes was corrupted...
...but data durability has been overall excellent, and compares favorably to disk.
Which is great, until the heads wear out. I would hesitate to buy a used drive unless the heads have been replaced, especially since LTO-4 is about 14 years old at this point.
