Jean-Michel,
if you search SCN you may be able to find posts about people successfully challenging AWS bills. i complained about $.01 and they reversed, albeit relatively small, monthly bill for me. you can also set up billing alerts, but if you don't pay attention to the line items on your bill and don't understand what is driving AWS cost, you may end up with a hefty bill.
they key is not to keep large amounts of data "hanging" in live running instances and get into the habit of turning off services that you don't really use. knowing which geo is really efficient for you helps as well as i have learned this month (the hard way of course).
have fun with the cloud,
greg