Ask HN: Help price my SDK?

Can anyone point me in the right direction of where I can find some examples and ideas on how to price an SDK for iOS/Android that charges based upon active users/month ? Right now, I'm thinking around 0.02 / active user / month, but when I look at some of the larger more popular apps, it seems unfeasible to charge this much ? What are the average active user % of total downloads usually?<p>Thank you!<p>Roland