App store rankings is the new and competed marketing area. Developers around the world are trying to find a way to increase their visibility to drive downloads. There are many factors influencing the app ranking. Some are textual like the words you use. Some are popularity based factors like number of downloads, uninstalls etc. Some are based on rating and reviews the application has received. Each area offers different opportunities for developers and each area should be optimised. But the key factor still remains the most recent download volume.
In order to boost your app store rankings, you must generate strong peak of downloads that hits a 72-hour window. This normally means advertising. The investment required is dependent on the country and app category you are targeting. USA is one of the most competitive market, while smaller countries get you up faster and cheaper. Of course the benefit will also be smaller.
The idea behind buying your way to the top of app store rankings is that the organic downloads will also increase dramatically. Take a look at this great infograph, created by TradeMob