...
Upfront cost - devices itself, including explicit network for storage (fixed cost)
Repair/management costs + cost of space (fixed over a long period of time)
Energy cost (usually fixed over a long period of time with its seasonal spikes)
Optional license cost when the commercial distributed storage system is applied
When added together summed up and divided into monthly payments over a time period that equals server units service life, by far, the highest one is the energy cost. To conclude, although it seems wiser to make use of old server hardware for storage clusters, it is actually not so at all. Much wiser is to buy new, specially configured, low power consumption hardware that may even come with installed and configured distributed storage systems. Such specially configured devices offer another benefit - fast cluster extendability.
In a typical distributed storage solution, there are implemented both object and block storage, giving in such a way an opportunity to (when implemented as separate clusters) to use object storage also as a base for backup or disaster recovery storageimplementations, in addition to its main purpose.
...