App store revenue is not an ideal way to scope the value of an ecosystem to developers. The majority of the revenue comes from games, mostly freemium using IAP, while a large proportion of the most valuable apps are offered for free and generate revenue through other means (Facebook or Amazon, for example).
However, it does give a pretty good proxy for the broader behavior of the users, and it also of course is very relevant for developers who do want to to charge.
For the first time, Google gave numbers for app store developer revenue at IO this year, and in the latest quarterly results Apple gave (almost) like-for-like numbers:
Google said it paid out $5bn to developers from Google IO in 2013 to Google IO in 2014 (a little over 13 months)
Apple said it has paid out $20bn to developers in total by the end of the June 2014 quarter, and at WWDC June 2013 it gave a figure of $10bn paid to developers (at the June 2013 earnings call a month later it then said it had paid out $11bn). So in the last 12 months, it paid out roughly $10bn.
Google also said at IO that it has 1bn 30-day active Android users - the degree of precision is not clear. The iOS number is fuzzier: trailing 24 months' sales would be a little under 500m, but extending that to a three year lifespan would take it to over 600m.
Obviously all of these numbers are rounded and were given at scheduled events, so need to be taken as imprecise. The fact that four different growth rates are involved also makes calculating ARPUs a little tricky.
In the last 12 months, on public numbers, Google has paid out roughly half of Apple - $5bn versus $10bn, on roughly double the number of devices.
On a run-rate basis, annual gross app store revenue across iOS and Android is now $21bn.
The chart below shows the public data points.
The problem with this, of course, is that with only two data points from Google, we don't know the trajectory - if this is a steep curve the recent period might be pointing more sharply upwards.
A further observation: if the current market dynamics remain, Google Android's user base will at least double in the next few years - the iPhone base is still growing, but it will probably not double. However, those users will be gained at progressively lower (much lower) device price points, and with significantly lower spending profiles.
For more discussion of why the two platforms look different, see this post.
Finally, just to make life easier, Play is not the only payment system you can use on Android, even Google Android. A material number of apps, mainly games and mainly in emerging markets, use other payment methods. So that number might really be somewhat higher.