It is hard to find pricing for the equipment, which makes me think it is expensive.but it could still potentially be cost effective. However it seems unlikely to come in at 10% of Amazon, ie at $150 per TB per year. You might be better off building your own robot for this application too, with more storage space.
Spun down hard drives would be a similar price. Not necessarily easier to work with.
I think spun down HDs would be a lot easier. They have standard interfaces which makes them cheap and flexible. Get a bunch of USB 3 docks[1] and some $10/hour clerks to shuffle the drives around.
Forget the clerks and USB. Get the very cheapest USB to ethernet chipset you can, on 100 mb ethernet, and only power it up when you need drive access. You should be able to do that at $10-20 a disk. No moving parts to maintain.
Spun down hard drives would be a similar price. Not necessarily easier to work with.