ANSWER:

Unfortunately this is less straight forward than one would think, especially given that the desired data can be inspected manually via your account. There are two monitoring options one would expect:

• notifications via email/RSS/etc.
• API access to the data

Neither AWS nor any other IaaS/PaaS/SaaS vendor I’m aware of does offer API access to their accounting data currently (maybe due to the potential financial/legal implications), making any form of 3rd party integration (which would be easy to do nowadays) cumbersome at best, insofar you need to resort to web scraping to retrieve the data in the first place.

Fortunately a new offering from Cloudability has entered the stage recently to do just this for you in a professional and vendor agnostic way, we are using it with great success already for AWS specifically – you’ll currently receive a daily (or less frequent) report of your monthly spending only though, i.e. not broken down to your daily spending yet. Adding the daily increase would be trivial of course, so I’d hope and expect they’ll make more information like this available over time.

Their approach to pricing is refreshing as well (despite being obvious) and simply tied to your own cloud spending, thus should pay for itself as soon as you realize respective saving potential (they don’t charge anything at all if you spend less than $2.5k/mo).

Update

The former caveat (kept for reference below) of requiring your main AWS credentials doesn’t apply anymore – AWS recently introduced New IAM Features: Password Management and Access to Account Activity and Usage Reports Pages:

This new feature allows you to create separate and distinct IAM users
for business and technical purposes. You can grant your business users
access to the Account Activity and/or Usage Reports pages of the AWS
website to allow them to access billing and usage data without giving
them access to other AWS resources such as EC2 instances or files in
S3

Cloudability has now integrated this as well, thus you don’t need to hand them your main AWS credentials anymore or spent the extra effort to establish Consolidated Billing just to gain insight into your cloud spending, see How to Setup Amazon IAM (Identity Account Management) for details.

Former Caveat

There is one caveat one should be aware of upfront though:

In order to access your data you’ll need to hand them your main AWS credentials, because otherwise they can’t scrape your account, obviously. For AWS in particular you can still avoid this by facilitating Consolidated Billing, where you consolidate payment for multiple Amazon AWS accounts [...] by designating a single paying account, which in turn has no access to your computing resources and data.

by Steffen Opel from http://serverfault.com/questions/350991