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Re:Ask HN: How did you significantly reduce your AWS cost? by rdlowrey on Monday October 30, 2017 @03:49PM (#15588285)
Spot instances. Spot instances. Spot instances.
Re:Ask HN: How did you significantly reduce your AWS cost? by matwood on Monday October 30, 2017 @03:53PM (#15588317)
This, this, and this again.

I'll also add, do not keep large EBS volumes around. If you need to store data forever, get it moved to S3.