The cheapest I can find for a consumer buying e.g. 20TB Seagate hard drives and rotating them every 3 years or so is about $5 per TB-year, without mirroring. So if raw storage cost optimization is what you're after that's what I'd go for to start. Even AWS Glacier doesn't come close to that, although you do get other things with it.
through you want at least one backup of yours to be off site, and your want your backups robust, so comparing hard drive cost seem strange as if you run the backup server yourself you need a decent raid and for the offline backup you need to compare with idk. S3 storage cost or similar
it's still more expensive but if you only need to backup some folders of documents or similar it might anyway be the simpler and cheaper solution
if you want to backup huge photo/video/vm image collections it probably isn't the best choice for you
but if you need to backup you photo
I think tarsnap was a good service about 20 years ago when it had little competition, but using it now makes very little sense IMHO. You can donate to its awesome FreeBSD maintainer, or to FreeBSD, directly.
Borgbase had a week long (IIRC) outage due to a failed attempt to add new drives to an array. As far as I know they never published a post-mortem on this and have never discussed how they're going to improve their disaster recovery so it can't happen again. It's difficult to recommend when they could leave you without working backups for an entire week.
Also you can back up to the hard drive under your friend's bed, and they can back up to the hard drive under your bed.
If you're even slightly technical, or have a friend who is, I'd recommend both of you buying the cheapest Kirkwood NASes you can find on ebay, throwing Debian on them, and becoming each other's backup buddies.