Spot Instances enable you to bid for unused Amazon EC2 capacity. Instances are charged the Spot Price set by Amazon EC2, which fluctuates periodically depending on the supply of and demand for Spot Instance capacity.
For batch processing, this new system allows you to trade urgency/throughput for cost. If you have some non-time-sensitive computing, you can wait until demand is down an the cost is low.
The price started climbing after they announced the service, but is still only about $.50 per hour. The regular On-Demand Instances price is $1.20 per hour. As more people use this service, I expect the average price to rise and to start to show daily, weekly, and yearly cycles.