The developers of Camera+ have been releasing sales figures throughout the years that Camera+ has been available. Of course, they make millions from the app so it paints a slightly different picture than the typical "indie" app sales numbers.
Some things to note about Camera+ sales figures and how popular their app has been: 1) they run the extremely popular MacHeist promotion so they already have an established audience and email distribution list of over 100k people, 2) they were well known in the iOS community before releasing Camera+ so they had a base of people to spread the word, 3) Camera+, when released, was an amazing and different camera app and the market loved it.
Camera+ first month: http://taptaptap.com/blog/cameraplus-first-month-sales/
Camera+ hits $2 million: http://taptaptap.com/blog/cameraplus-hits-2-million-sales/
Camera+ hits $6 million: http://taptaptap.com/blog/cameraplus-reaches-6-million-sales...
Some things to note about Camera+ sales figures and how popular their app has been: 1) they run the extremely popular MacHeist promotion so they already have an established audience and email distribution list of over 100k people, 2) they were well known in the iOS community before releasing Camera+ so they had a base of people to spread the word, 3) Camera+, when released, was an amazing and different camera app and the market loved it.