>8TB hard drive is about 200 dollars nowadays, so this amount of data is about 1.6B dollars worthy of retail hard drives.
External hard drives can be obtained on sale for $15-16/TB. They can then be shucked to get internal drives. I suspect bulk buyers can get pricing equal or cheaper than this, rather than the retail of $25/TB
Edit: I googled it an apparently it’s an economy of scale thing. People are more likely to buy external hard drives, much more mass market thing, Costco and Walmart sell externals, therefore downward pressure on prices compared to internal drives.
Do external drives even have performance/longevity specs? The specs I see on them are pretty basic (eg. capacity, interface, operating temperature), and considering that they're marketed to consumers that's not surprising. As for actual performance/longevity, my understanding is that they're just whatever drives the manufacturer has available. For low capacities they're going to be consumer drives and for higher capacities they'll usually be NAS/datacenter drives.
At the scale these data centers operate at, I would imagine that google / FB / AWS gets these storage devices custom built to whatever spec and tolerances they need.
External hard drives can be obtained on sale for $15-16/TB. They can then be shucked to get internal drives. I suspect bulk buyers can get pricing equal or cheaper than this, rather than the retail of $25/TB