What is the average user retention rate of free iphone apps and how does that compare to that of paid iphone apps?

I have submitted a number of free and paid apps. The free ones train spelling, and they show links to my paid apps, that train reading and writing. My experience is that the ratio between the free and the paid apps is about 6 percent. Each time I update the free apps, they drive some additional sales of the paid ones. The revenue is not great, but it has been pretty stable over the last year, in spite of not having done changes for about 7 months. The apps are stand alone, not in-app purchase. After I started submitting the free apps, the sales of the paid ones increased by a factor of 5. Right now I work on new and improved versions of both the paid and free apps. They will be updated to the iOS7 level, have new graphics and sentences and use animations. This will make them more interesting to work with. Furthermore, my future apps will be universal, running nicely on iPads, iPhones and iPod touch devices. They take longer time to make, but they surely are also much easier and faster to maintain. About 30.000 of my free apps that people already downloaded should work nicely as (free) advertising.