I use rsync.net's borg offering, which is similar (encrypted + deduplicated) but 17x cheaper (1.5 cents vs. 25 cents per GB-month, and also zero bandwidth fees). I think it's even less with the HN discount; I'm too lazy to check.
I pay $150/year for BorgBase backup and store 1.66TB, mostly because I really like Borg backup and am comfortable with it. That's my last-tier backup. I highly recommend BorgBase, they're good folks.
I also pay $120/year for Google drive which is my "online" backup, then store the files locally of course.
Seems to work OK for me, and I'm insulated against the problem of: "google's algorithm decided I was a Bad Person and terminated my accounts".
I can't follow your math. If I had 40 GiB, it would cost $10.00/month to store them on Tarsnap (40 GiB * $0.25/GiB-month), or $1.50/month to store them on rsync.net/borg (the 100 GiB minimum order).
https://www.rsync.net/products/borg.html
To be completely fair: since Tarsnap doesn't have a minimum order size, it's still a better value in the 0-6 GiB range.