Amazon’s spot market for computing power is set up as an open market for surplus servers. The price is dynamic and depends on demand. So when demand is low, you can get computing instances for rock bottom prices. When you do that you normally set a range of prices you’re willing to pay. If it goes over your top end, your instances get killed and re-provisioned for someone else. Obviously this wouldn’t work for all applications, like a website that has to be up all the time, but for computing power, say to run some huge hedge fund analytics, it might fit perfectly.