Note that the BackBlaze machines are optimized for very cold data since they only need to support backup and restore. We also do custom hardware at SpiderOak, but we support web/mobile access, real time sync, etc. That makes our hardware slightly more expensive because of the generally warmer data. So you're off by a few pennies, but certainly in the right zone.
For Amazon, I suspect their internal S3 cost is actually quite a bit higher than either BackBlaze or SpiderOak since their data is warmer.
What much warm data data do you normally have/node?