Using Spot Instances can reduce costs by at least two times as compared with On-Demand instances.
On-Demand (Large): $0.48 per hour
Spot Instances (Large): $0.20 per hour
Set your maximum price near to the current Spot Price. If the Spot Price goes above your maximum price and your instance is terminated by Amazon EC2, you will not be charged for any partial hour of usage. Just imagin, you have completed the tests in 30 minutes and the instances are termintaed by Amazon EC2. You will get testing for free.
BTW: Do not tell anyone about this;)
Have requested the large instanse for maximum bid price $0.250. You will often pay less per hour than your maximum bid price. The Spot Price is adjusted periodically as requests come in and available supply changes. Everyone pays that same Spot Price for that period regardless of whether their maximum bid price was higher. You will never pay more than your maximum bid price per hour.