“A basic Azure account gives you 20 storage accounts and each one can have 200TB of storage. The on-prem equivalent would costs you millions."
According to http://www.windowsazure.com/en-us/documentation/articles/storage-whatis-account/, you only get 5 accounts. Still, that is a whole lot of storage. Wonder how cheap it is in the cloud? Local replication only as that is the cheapest option:
200TB * 5 accounts = 1000TB = £0.045*1*1024 + £0.042*49*1024 + £0.039*450*1024 + £0.036*500*1024 = £38,556.67 per month for storage, plus a fee per read or write, plus data transfer fees if you're going out of the cloud.
As a slightly silly comparison, for the same money you could buy 1080TB of new storage ($64357 / $59.54 per TB in backblaze storage pod 3.0s) each month, and then it is yours forever (after month 2, you have local redundancy). Even with the power and cooling costs, you're not going to save money putting bulk data into the cloud.
The advantages of the cloud are correctly sizing instances to loads, rapid scaling, ease of management, and reliability. As long as it is easier and cheaper to spin up a new instance than it is to provision a new VM, and reliability is comparable, cloud makes sense. Mass storage doesn't, so far, seem sensible (even ignoring the vendor lock in potential of having all your data held by a 3rd party).