As per https://www.backblaze.com/b2/storage-pod.html they are not using enterprise drives and 15 drives per RAID6.

As per https://www.backblaze.com/blog/open-source-data-storage-serv... it costs them $0.036/GB.

So 8TB space costs them 294,91$. Well they have other costs too. But if they had him as a customer for 5+ years, they can start profiting :) Actually more years, because electricity, traffic, rent, salaries, etc...