If you do the math, you see that an 8TB drive can be found for $125 (granted, it's a special deal) at https://slickdeals.net/f/10919999-8tb-seagate-backup-plus-hu... - but it's quite common to find them around $200. Then the RPi 3 is under $50.

Without even talking about the difficulty of dealing with Glacier files in an incremental backup scheme, doing the initial sync, and checking the backup data regularly, we're talking about something that will probably cost $250 for a couple of years of backup (assuming the drive only lasts 2 years) vs $720 (glacier) or $2,400 (S3) if I use your numbers.

It seems like a significant difference to me, especially because the assumptions on how often my drive will fail are quite pessimistic.