A couple of years ago I was confronted with a bill of several hundred dollars because I’d forgotten to turn off some machines on AWS ( I think it was an ELB – elastic load balancer). Since then, I make it a point to login and check often to see if I’ve left stuff running. I’ve automated this simple check here: https://github.com/chiradeep/idle-instance-reaper
You can run the check using AWS Lambda as well. Just make sure you configure a ScheduledEvent trigger for it.
Hope you save some money with this tip.