Microsoft has announced the availability of two new REST based APIs for accessing billing data.
Azure Usage API – A REST API that customers and partners can use to get their usage data for an Azure subscription. As part of this new Billing API we now correlate the usage/costs by the resource tags you can now set set on your Azure resources (for example: you could assign a tag "Department abc" or "Project X" to a VM or Database in order to better track spend on a resource and charge it back to an internal group within your company). To get more details, please read the MSDN page on the Usage API. Enterprise Agreement (EA) customers can also use this API to get a more granular view into their consumption data, and to complement what they get from the EA Billing CSV.
Azure RateCard API – A REST API that customers and partners can use to get the list of the available resources they can use, along with metadata and price information about them. To get more details, please read the MSDN page on the RateCard API.
You can write custom solutions for reporting, analyzing and predicting customer billing data.
There are code samples available on GitHub.