Your bill is calculated from size your backups currently storaged are using. It depends on your database size, the frequency you're backing it up and your retention policy.
At the end of the month, this average cumulative size of all the backups during this period will be used to calculate your bill. The first 10GB are included in $5 flat fee for the month.
For example, if your database backup (compressed and without indexes) weights 500MB, you backup it twice a day and keep the backup for 2 month, the total storage used will be: 0.500GB × 2 backups/day × 60 days = 60GB Your bill will be: $5 + ( 60GB - 10GB ) × $0.30/GB = $20 / month