Just the sheer number of instances the OP is using has put the usage FAR into the territory where the premiums spent on virtualized instance costs far outweigh any clever strategy one might use to make things cheaper in the cloud. It has nothing to do with what the infrastructure is, and everything to do with raw volume.
Spot instances are super cheap...about 80% less than regular instances.
Yea but it's still virtualized slow molasses at the end. And still more expensive. My point is that at 25k servers, even a 5% efficiency gain is significant, and I'm willing to bet the margins are still higher.