Repeating a comment I made above - for standard tier, requests are expensive enough that it's cost-effective to let space on the disks go unused if someone wants an IOPS/TB ratio that's higher than what disk drives can provide. But not much more expensive than that.
The latest generation of drives store about 30TB - I don't know how much AWS pays for them, but a wild-ass guess would be $300-$500. That's a lot cheaper than 30TB of SSD.
Also important - you can put those disks in high-density systems (e.g. 100 drives in 4U) that only add maybe 25% to the total cost, at least if you're AWS, a bit more for the rest of us. The per-slot cost of boxes that hold lots of SSDs seems to be a lot higher.
The latest generation of drives store about 30TB - I don't know how much AWS pays for them, but a wild-ass guess would be $300-$500. That's a lot cheaper than 30TB of SSD.
Also important - you can put those disks in high-density systems (e.g. 100 drives in 4U) that only add maybe 25% to the total cost, at least if you're AWS, a bit more for the rest of us. The per-slot cost of boxes that hold lots of SSDs seems to be a lot higher.