Some of below is going to be specific to Backblaze B2 pricing and screens, some is generic.
My Account → Caps & Alerts page will tell you where costs come from. It’s not only storage.
Downloads and possibly transactions can be billed when your use exceeds the free amount.
Reports page will give a longer-term view. Just looking at dailies may miss work of compact.
There’s some tradeoff. –no-auto-compact may cut download cost, but increase storage cost.
Running backups frequently won’t raise upload costs (which are free), but will do downloads.
B2 Buckets page gives size of all your buckets, if it does turn out that storage is the big cost.
Maybe you will find some things you no longer need at all now, but which you’re charged for.
Sometimes old backup versions get increasingly unimportant. Deleting some will save space.
Backup retention on screen 5 Options has a variety of options to keep versions under control.
Note the space will not immediately be lowered, as reclaiming the waste is done by compact.
Duplicati Home screen has the Source size and the Backup size for every backup configured.
Compression and deduplication can make first Backup smaller than Source files that it’s from.
Eventually version buildup will probably make it bigger. Detailed size information is in job logs.
Introducing Backblaze B2 Lifecycle Rules shows how to see if you actually store old versions.
This isn’t what Duplicati relies on, as many storage types don’t have it. Duplicati does its own.
My B2 Lifecycle Settings remain at the initial Keep all versions of the file (default).
Looking at names and icons on B2’s web UI, I’m not seeing any B2-level old versions present.
At Duplicati level, I’m using Custom backup retention similar to Smart backup retention.
New retention policy deletes old backups in a smart way was the announcement of the feature.