maybe I'm doing this wrong, but if I use RDS / Cloud SQL / Compose etc - a normal base cluster + another failover cluster + hourly/daily backups (based on your release schedule) all add up to around 30 - 50 based on various cloud providers.
For backing up postgres, all you have to do is setup a cron job that backs up the postgres' data directory to S3/Google Drive/Dropbox every hour/day.
If you want proper replication and failover then you can probably use 2 digital ocean droplets each for $5/month and another $5 VM for the application server itself.