It's cheap for blob storage. It's terribly expensive compared to hosting yourself.
So yeah, I'd agree with you. But for anyone prepared to use S3 for anything but cold storage, this is still a lot cheaper.
My suggestion would be to use this for cold storage + big cache boxes at a provider with low bandwidth charges. Especially if your "hot" objects make up a relatively small percentage.
Backblaze is the same cost or cheaper as the cheapest tier in several other popular services like Amazon S3 and Microsoft Azure. We don't know of anybody with a lower cost of downloads.