The starting point to this surely will be, please define your use of the term 'cloud'... Then, what exactly do you want to achieve and why?
Your question appears to suggest you will buy the hardware. In such case, a simple look at any number of suppliers for large scale networked storage will tell you the hardware cost starting point.
Your intended use will then tell you if extra hardware is needed (InfiniBand, drive buffers / caches for prefetch and read write, a hybrid system, etc), and then if you can even fill that much space... exactly how you will be reading and writing the data, and any performance and bandwidth allocation...
The more background you supply, the more meaningful a response can be given.
If you are just shooting from the hip, then the only answer to how much? Will be a lot / too much, depending on your budget...
C.
The cost of 1PB of storage is pretty hard to measure, because there are many factors. The biggest hard drives available provide 12TB of storage. So that mean you need 83 disks to have 1PB.
SSD drives are already 16TB+ for the largest production drives (Samsung), and in 2016
Seagate announced a 60TB that was to come out in 2017, but seems delayed... Putting 24 of these Seagates into a single chassis would give you around 1.44 PB as a JBOD.... and if there is no budget cap, and no rush, that may be the best option for the mother of all arrays....
C.
