Make it free and monetize it like all the free-to-play games do: If you want to cross of a non-top task, you have to pay increasing amounts of (real) money, e.

Make it free and monetize it like all the free-to-play games do: If you want to cross of a non-top task, you have to pay increasing amounts of (real) money, e.g. 2nd task 0.50 $, 3rd task 1$, 4th task 2$ etc.