I put a note about spot instances at the end (the economics of spot instances are a bit tricker for calculating cost-effectiveness due to price variability).
Although the cost of a K80 preemptible instance on GCP is now close to the approximate cost of a K80 spot instance on AWS, so there's a bit of competition.
Disclosure: I work on Google Cloud (and helped launch this).
Note that our pricing is flat regardless of number of GPUs attached (and you don't need to buy as many cores to go with them). By comparison, Spot often charges greater than on-demand pricing for anything other than single GPU instances.
Thanks again for your write up, sorry about the confusion as we delayed announcement until the new year.
Although the cost of a K80 preemptible instance on GCP is now close to the approximate cost of a K80 spot instance on AWS, so there's a bit of competition.