Having worked for storage services for the cloud, it's not a massive premium. Things are kept pretty close to costs. We have to, it's a similar race to the bottom case. Each time one of the clouds drop storage costs, the others rapidly follow even to the point of it losing them money while they figure out how they might return to profitability.

The biggest cost for cloud storage services is not disks. Never has been. They're pretty cheap all things told. It's the per rack operational costs that dwarf them. The cost of electricity, cooling etc. You're also paying for the durability and accessibility that is built in to the software. It's why you see, e.g. backblaze obsessing about their server specifications and how many disks they can cram in to the server. Everyone is trying to maximise the rack data density, and trying to tow a really fine line on having just enough compute power in the individual server.