"The best way to approach auction type situations like this is often to simply list the maximum price you can afford. Your instance(s) will get run if and when the spot instance price reaches that price and you will regularly get charged less depending on what other users are bidding for their instances.

Though I don’t recommend trying to chase the spot instance price around, it is natural to be curious about what others have been paying and whether or not you might have a chance to get in with your bid."

"Spot Instances are a new way to purchase and consume Amazon EC2 Instances. They allow customers to bid on unused Amazon EC2 capacity and run those instances for as long as their bid exceeds the current Spot Price. The Spot Price changes periodically based on supply and demand, and customers whose bids meet or exceed it gain access to the available Spot Instances. Spot Instances are complementary to On-Demand Instances and Reserved Instances, providing another option for obtaining compute capacity."