This $500 would have been $60, on a Digital Ocean box. DO has 1 TB/month limit. Their usage was 300 GB/month.
Digital Ocean is a great deal! Thanks for pointing that out.
The reason I use {aws,azure,google} to host my NTP servers is that my day job is developing a VM orchestrator (BOSH) for Cloud Foundry, and BOSH doesn't support Digital Ocean yet (AFAIK). But that's a personal choice, and an admittedly expensive one.
It'd be nice if BOSH could have detected this problem and warned us about it, ideally during deployment. But it'd be even nicer if we didn't suck at configuring AWS. If you could fix either of those things, that would be great!