It is great that AWS allows you to pay-for-what-you-use and as-you-go. Something to keep in mind, however, is that AWS meters usage in hourly increments.
So, say you were running a number of batch jobs, launching and terminating an instance every 10 minutes. After an hour and 10 minutes, you would have launched and terminated six instances (6x smallest increment of 1h) resulting in 6 hours of billable usage despite the fact the neither of them lasted more than 10 minutes.
At any rate, to avoid surprises, it is highly recommended you to set up billing alerts. These are simple CloudWatch alarms which can notify you when your estimated bill has reached a threshold.