Apple paid over 2 billion USD to their app developer community.
Today in app store (source 148apps.biz) :
Total Active Apps (currently available for download): 351,569
Total Inactive Apps (no longer available for download): 75,833
Total Apps Seen in US App Store: 427,402
Number of Active Publishers in the US App Store: 72,384
Free    apps    121,307 (34.50%)

So we have 230k paid apps. This is in average $7,000 revenue per app.
We have around 72,000 Active Publishers in the US App store. If 30% only offer free apps then we have about 50,000 publishers which made money with their apps, which would be in average $40,000 per publisher revenue, which is not too bad. You don't make a living but it is a nice income.

However the average revenue might be less, because the free apps have usually iAds and the $2 billion paid to developers do include iAds.

The numbers seem at the end not high, but still higher than right now the competition can offer.