[res0nat0r]

Set an SNS alert to sent an email/SMS message to your phone if your monthly bill goes over whatever $X you decide. I've had this set on my personal account for years and it isn't too hard to configure, most of it is just point and click via the SNS and CloudWatch GUIs and is pretty foolproof.

[jjnoakes]

From all of the horror stories I've heard, it is not foolproof. For one, don't you get notified after the usage and charge happens? So one mistake that causes a large spike and the notification is too late.

[res0nat0r]

This is true, it isn't really possible to get a near instantaneous real-time feed of every single charge from all of the different AWS services you may be using, because they are all unrelated and do their logging / billing differently. IE EC2 will scrape and upload your iptables data-usage info to s3 and then that will get scraped and generated into a daily billing/money report etc, and there are thousands of things such as this between all of AWS services.

This likely will just alert you somewhat quickly after something has spiked and been running for a number of hours/day, most likely.