Running CC2 as an on demand instance type you're going to pay $2.40 for the right to use the 32 core system for just 1 hour. However, if you're able to create your application in a way that takes advantage of AWS Spot Instance running on a SUSE Linux distro, you can run the same application for as low as $0.0001 per hour (yes 1/100 of a penny) quite regularly. Over the course of the last week you would have paid only as much as $0.10 per hour with most of the usage coming in at nearly free.
I'm sure you can find more instances being offered at such a low cost. This was just one example of a very expensive instance being offered essentially for free.
Of course, you've got to be careful with spot instances, they can be terminated at any time. You'll find these work best when application state and results are stored perhaps in a dedicated instance. Some great candidates for spot instances may include idempotent operations and reproducible or parallelisable work. You can see that running a CI server would be well suited for this work. You don't really care if the instance shuts off unexpectedly, it can resume later and produce your build/test result successfully at a later time. Processing work for the likes of the good 'ol SETI project would also be a great use case.
Happy cloud computing!