IIRC bucket shall be public, it was long time ago I played with this. This does not matter though – you data is encrypted anyway.
B2 however is still hot storage. They are competing with Amazon’s hot storage and therefore they can claim they are “cheaper”. For backup hot storage is an overkill, by using even cheap hot storage, you are still overpaying for access performance you don’t need
Consider Amazon cold storage tiers instead. While duplicacy does not support ultra-low cost archival tiers – even instant access ones can provide better cost structure.
For example, S3 Intelligent - Tiering, and S3 Glacier Instant Retrieval costs under $0.004/GB. And you have 100GB/month of free egress
With amazon storage the concern is usually - what if I have restore everything? The restore cost will be astronomical. Yes, kind of. Restore is expensive, if you want all data back asap in its entirety. But this cost must be multiplied by the probably of needing to restore. Which is very low. Ideally, you never will have to restore full backup – but you pay for storing it every month. Hence, taking a hit on restore cost but optimizing the storage costs makes sense.
For small restores, such as revert few files, etc – 100GB free egress per month covers that.
I’m not suggesting to blindly switch to amazon though – you need to be careful about the cost and plan accordingly, but with the right planning this can be very cost and performance effective.
If you go hot storage route – consider Storj, instead of backblaze. The cost per TB stored is lower than B2, egress is lower, you get 150GB monthly traffic and storage free, and, more importantly, this is distributed, geo-redundant storage. I really like it, consider it