I don't really know much about optimizing storage costs, But You could learn from storage giants.

Example is Blackblaze storage pod 6.0 according to them it holds 0.5PB with a cost of 10k$, you will need about 20*10K$ = 200K$ + Maintenance(They also publish failure rates) , The schematics and everything is in their website and according to them they have already a supplier who provides them with such devices which you could probably buy from. Note: This was published 2016, they probably have Pod 7.0 by now so cost may be better.

Reference: https://www.backblaze.com/blog/open-source-data-storage-serv...