Then again, the title of the post says "practically free" and goes on to say it's actually $5 a month, so I took the rest of the figures with a grain of salt...
B2 itself is not $5 a month. That $5 service is a separate consumer-focused "unlimited" backup service from the same company. B2 just charges for storage and transfer. Yeah, the author was rambling a bit.
B2 costs me roughly 52 cents per month to backup 250GB of data. I'd say that if you have enough income to have an internet connection and generate enough data to want a cloud backup tool, it is effectively free.