While many people complain about in-app advertising and would like an option to buy, the numbers show that the consumer behaviour is the opposite. On the iOS platform (regarded as the mobile platform that has the most sales on a 'per app' basis) 2013 shows over 90% of the app store are free apps, and 6% are in the lowest 99 cent band.
Looking at the numbers for another side, Flurry examined apps where the developers had been doing price comparisons over time (A/B testing) - after testing more than half of developers went with a free app (65% in 2010, up to 80% so for in 2013) - presumably to keep the demand and download numbers high.
Flurry's graph on app pricing.
Traditionally Windows Phone income levels per download are somewhere between iOS and Android, but the message across the platforms from users is clear - while niche apps might still command a decent purchase price the mainstream apps are turning to other models, including in-app advertising and purchases.
You can read more, and see all the graphs, at Flurry's blog.