I had to double check just in case, but petabyte is only 1000 terabyte. It may be big in terms of database, but rather small in absolute terms. You could fit a single Petabyte in a 1U server.

I doubt they pay listed price. And AWS is now mostly a Enterprise and Sales game. So once you ran other cost involved in managing, I would think you need to be multiple Rack scale before the cost break down better for your own hardware.

And that is excluding other benefits of sitting inside AWS ecosystem. The only thing I think AWS isn't so good at is the low cost, sub $1000 per month spending scenario. Where you are paying a lot more just for staying inside the ecosystem for things you may not be using. Those tends to flavour Linode or DO.