I was also seeing a surprising cost of about twice what I expected, but since I rarely use AWS I was assuming I had just calculated it wrong somehow. It's maddeningly difficult to see your usage history of services -- I still can't figure out how to see what my EC2 usage was this month. There is something called the "cost explorer" which I can't use yet, because apparently it takes 24 hours to set up (?), and it's not even clear that it will include my historical activity when it's ready...