Storing a 250GB backup and sending to 10 machines: $1 for storage, $22.5 for egress.
Storing a 250 GB backup, 2250GB of /dev/zero, and sending your backup to 10 machines: $10 for storage, $0 for egress.
It seems incentives also depend on the nitty gritty details around how you are billed which are not defined very clearly. Granularity and timing, whether you pay for part months of storage etc.
However all games aside it does seem like if your own egress were free (or much cheaper than 0.01) it would be most efficient to send your data directly to where it needed to go, yourself.
Then you'd only be paying $1 for the backup.
That said, it almost breaks even after the first month and pays off for itself a week into the second month. To eliminate this incentive, I'd suggest setting the cost of egress at (or below) the cost of storage.
It also would make more sense to me as a prospective customer that I'm not paying crazy overage fees, and I'm paying as if I reuploaded the same data and downloaded it an extra time.