Peter Farago, in an analysis for Flurry:

We start by taking the revenue generated in the iTunes App Store and setting it to 100%.  We then compare the relative revenue generated from Amazon and Google to the amount of revenue generated by the iTunes App Store.  Doing so, we find that Amazon Appstore revenue is 89% of iTunes App Store revenue, and Google Play revenue is 23% of iTunes App Store revenue.  Note that this is analysis shows revenue generated per active user. Another way to interpret the results is that for the same number of users per platform, every $1.00 generated in the iTunes App Store, will also fetch $0.89 in the Amazon Appstore and $0.23 in Google Play.  These results mirror those of a similar analysis conducted by Flurry last December, where we found for every $1.00 generated in the iTunes App Store, developers generated $0.24 in the Android Market.
Ouch.