I've dealt with hundreds of hard drives over the years. Using them as offline storage is very risky as they do and will fail to spin up after being sat for long periods of time.
You want to take some redundancy into account. Even with that, it's still cheap.
Tapes degrade and break down as well. Everything needs maintenance. If you want really long term storage, there is no other option but to copy the data to new mediums once in a while.
Most of my experience has been with Ultrium TBH. But you can pick up Ultrium 1.5TB tapes for less than the cost of a equivalent sized hard drive, here in the UK at least.
That's because disks are expensive in the UK, not because tape is cheap.
You can get 2TB of disk storage for <$100 in the US when you are buying retail (if you buy three hundreds like this guy needs, you'll probably be able to get it down to $30 or $40 per TB).
No tape device required, comparable speed, disks are random access; A backblaze-style setup in todays costs sets you back ~$60/TB for online, random access, storage redundancy and other nice things - 500TB can be build for $30K or so, with a few hundred dollars/month for electricity (and a few hundreds more for colocation if you don't have the office space).
How cheap can you get a functioning system with reasonable (a few minutes) retrieval time when you have 400 1.5TB tapes? (you need redundancy here too, you know).
Since sometime in '98, tapes make no economical sense whatsoever for any kind of storage, and hardly any sense at all, as far as I can tell.
Does it need to be online at all times? If not, why not use offline drives and store them somewhere?