Brian from Backblaze here. Our new B2 product is priced at half a penny per GByte per month which accurately reflects how much it costs for us to store your data including a small profit for us.
So the $5/month is profitable for us up to a 1 TByte backup. We have about 25 customers with more than 50 TBytes in a $5/month backup, and yes, we lose money on them (which is FINE - they often recommend us to friends with less data). On the opposite end, we have about 20,000 customers with less than 20 GBytes backed up where we are massively profitable on those particular customers. Interestingly enough, my 84 year old father is in that demographic - no digital music, no digital movies, a few digital photos and a Quicken file. Last year he lost a hard drive, we restored his files from Backblaze. :-)
In between the 50 TByte and 10 GByte customers is a big bell curve with the bulk of our customers basically paying for their own backups.
A different way to think about it is the vast majority of people store files inside their laptops and are happy. The maximum size hard drive you can configure in a laptop today is probably about 1 TByte (the 2 TByte laptop size drives are just starting to appear), so by definition we're profitable on people like this. Technical people think everybody is like them and has a 16 TByte RAID array filled with all the Linux ISOs and movies. :-) But really most people have less than a TByte of personal data.