From an article with catch title, Are iPhone Apps a Failed Model, that I read:
2H 2010: total Apple App Store Sales in the second half $858 million; annual sales calendar 2010 $1.4 billion
With 225,000 Apps developers the per app developer the payment comes around 6.5K which is a very small amount to make a business out of it.
However, not all apps developers get paid or depend on paid App. There are four additional monetization models:
1. Ad Revenue: Many games and entertainment apps depend of ad revenue
2. Transaction Revenue: Apps like myBantu depends on transactions to make money
3. Virtual goods: Games and other apps use virtual products to make money
4. Client Apps: Client Apps like enterprise Apps are just access but the money is paid to the enterprise licenses.