My understanding is that objects in S3 Glacier and Deep Glacier classes are in general not accessible in real time. This model does not work well with restic, which expects real time access to repository contents.
I learned this the hard way a couple of years ago when I unintentionally set up a rule that automatically transitioned objects to Glacier. Once transitioning began, restic operations on the corresponding repository (understandably, in retrospect) failed completely because it could not access the transitioned objects. Possibly things have changed since then, but that was my experience.
FWIW, restic works fine with objects in S3 Standard Infrequent Access. It’s not as cheap as the Glacier classes, but it is cheaper than Standard and (the last time I checked) competitive with Backblaze B2, which I also use. I transition repo objects to SIA automatically after 60 days and although I do incur some charges associated with early access when I prune they are small, typically a few cents per month.
I’m not sure how to address your questions around cost, as the answers will depend strongly on exactly what you do and how you manage your repositories…a prune operation will generally result in much higher data usage than a backup operation, for example. That said, just to give you an idea, my backup situation is quite similar to yours in terms of size and monthly activity, and I spend about 4 $/month on S3. I run daily backups and a monthly forget/prune. That’s with most of my data having transitioned to SIA; it would of course be more if my data lived in Standard.
There are other factors that you may or may not consider important. One is speed. For me, S3 is much faster (maybe 5x faster) than B2. This may be only my location (the AWS data center I use is much closer to me than than the B2 center), but it is what it is. Another is durability. AWS promises higher durability than B2, and depending on your level of paranoia you might care. (I should emphasize that I use both services and that I’ve never lost a bit with either; it’s just a question of how many 9’s you want to pay for).