I just started using Backblaze B2, switching from AWS S3. I use it through Rclone to encrypt everything before I upload it. Only $6/TB/mo and it only charges what you use. You can download up to 3x your data stored per month for free, so if you ever decide to move or you need to restore a backup, you don't have to pay egress costs. It was the cheapest service I could find, but there may be others.
You can set price limits on your account to ensure you don't go over too. It will alert you when you approach the limit. There are some minor costs, like pinging the API, but they are free up to a reasonable amount.