Yes, it was AWS S3.
I don’t have the exact numbers unfortunately as I can’t see the S3 data usage more than 1 year old in Amazon’s interface, but as I said I think the order of magnitude is right.
I do know that my backups were ~30 GB each, and that I was keeping about 3 backups max. As for the frequency, I don’t remember if it was once every two days, or every week.
I still have the bills, and they say between 5 and 7$ a month. They don’t show any data-related information.
I stopped using s3 because of the cost. That was not that expensive as you say, but I was trying to balance costs between the different services I subscribe to (hosting, emails, CDN, backups…), and decided to sync my backups to my Google Drive with rclone instead, for free.
The drawback is that while I trust Discourse for using S3 reliably, when using rclone, I do not have as much trust, and I must ensure the backups are properly synced to Google Drive. And I continue to monitor it from time to time, especially since I’ve noticed that at least once, the Google token wasn’t properly refreshed, and my backups stopped being sync with Drive.
I might change the way I manage my backups in the future, I don’t know.
Yes, it is for Discourse 
Backup size was like 27 GB two years ago, and now 30 GB.
Like many instances, most of the backup’s size is uploads. Uncompressed database is 23 GB, tho, but text is efficiently compressed.