How does Apple AppStore rank search results? I have an iOS application - an alarm clock. I released it a little over a month ago. Initially it was paid, but when I saw there were virtually no purchases (from 0 to 4 a day), I changed it to be free for a period of time. When it become free, in less than 10 days it has been downloaded more than 30,000 times, and more importantly (I think) it moved much higher in the search results when the user searches "alarm" - it reached 20th place in the results. During that time I also received very good reviews in the AppStore. But today I changed it back to paid, and now when I search "alarm" my app is shown at the bottom of the 4TH PAGE. Given that one page contains 120 apps, this is very bad for me and I don't understand why this happened. Can anyone tell me how does AppStore ranks search results and is there a way to move my app higher in the rankings, please? :) Huge thanks!
I'd venture to guess and say that the App Store ranking algorithm is a company secret, much like Google's web search rankings. It seems to me that the radical changes in placement you are witnessing are as much the fickleness of App Store customers as anything else. 

Correlation isn't causation by any means, but it seems fairly clear by your description of the events that if you want a lot of downloads you will either have to make it free (and perhaps ad-supported?), or your alarm clock app needs to do something so unique amongst the plethora of alarm clock apps that customers will feel compelled to pay for it.