I had exactly the same issue when starting this course and since then I have switched to spot instances because otherwise bill is growing fast. Especially when the instance is running but also when stopped, due to the reserved volumes and IP-adresses as @Will noted.
It is a bit more effort to use spot instances, but I think it is worth it in order to control charges. Here I made a description of how I set it up and then terminate it after every use:
http://forums.fast.ai/t/thoughts-on-spot-instances/156/21
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