There was a bug in our code which lead to a large number of invocations in lambdas which costed us the next 6 months of budget. We are talking to the AWS Support to check if they can refund the same. However, how do you guys proactively monitor costs?
Use the billing alerts feature of the CloudWatch Alarms subsystem.
If you get an alert saying that you've hit fifty percent of your monthly spend-target before the 15th of the month, you know that something's gone awry. =)
Yes, this is a useful tool; but the Tool suggested in the next answer seems to be of more use. However, thanks for helping! :)
We faced a similar issue and created a solution that we use in house. We got clients who had the same issue, so we made it part of our product.
You can use a template "daily lambda cost predictor".
Thanks Veer.
This seems to solve my problem to great extent.