My rough numbers are 450GB stored monthly, 4GB downloaded monthly, 90k stored files, 385,000 individual transactions, which ends up costing about $2.25 in storage fees, $0.25 for transactions, and $0.10 for download bandwidth.
I've been very happily using Restic and B2 for a long time. It's cheaper than the unlimited service given I have ~600GB stored. Plus, it's a "real" backup: on-going series of snapshots, older files are not replaced when a new back up is made, I can restore from as many points in time that I want to store, and I don't have to worry about Backblaze deleting anything due to inactivity.
One underrated feature of Restic is tagging, which lets you identify a collection of snapshots. I can point multiple separate backups and devices to the same repository tracked with tags; thus, I de-dupe across all of them.
If I'm reading this correctly, Restic + B2 sounds like an absolutely godsend!
Some more (informal) analysis on restic's crypto was done here: