Is there an API to get sales report on Google Play?

Google Checkout (which is used by Play for billing) has an API, but you need a merchant key to use it, and it is not available in all countries.

https://developers.google.com/checkout/samplecode

As someone pointed out below, Google Checkout has been discontinued. The new Google Wallet site does not offer a real API, but you can download your reports, which are stored on Google Drive, using the Google Drive API or command line tools. Reports are in CSV format, so they are easy to parse.

Additionally, the Google Play Developer console now shows sales data. Checkout the Andlytics app, it can fetch and display sales numbers (available in the alpha version). It is also open source.

https://play.google.com/store/apps/details?id=com.github.andlyticsproject

Edit 2014/02/18

Wikipedia

Google Checkout was discontinued on November 20, 2013. The company offered a new solution for certain payments called Google Wallet

I only have seen Apps that show a webview of the Developer Console. I noticed the URL to the statistic is always the same, year + month, so you could try and read this in. Sadly it only shows sales and not downloads. https://play.google.com/apps/publish/salesreport/download?report_date=2012_05&report_type=sales_report&dev_acc=%%YOUR_ACC_NUMBER%%

But there seems to be some kind of api when you look at the ajax-requests in the developer console: https://play.google.com/apps/publish/statistics?dev_acc=%%YOUR_ACC_NUMBER%% POST: method -> "getCombinedStats"