The first question I have is why would anybody pay for something they can get for free... Unless the free "whatever" is so difficult to get and free ends up being a lot of money (ie time).
If you have a value proposition to take away the pain, then that's a start.
Once you have established value, then you can determine who is willing to pay what for it.
Unfortunately, the question lacks details to provide a substantial answer. I can't help but respond to this question with questions of my own:
- In a general sense, what need does your app fulfill?
- What properties distinguish your app from the free apps?
- Does your app process Personally Identifiable Information?
- Have you performed a risk analysis on your app?
- How much are you willing to spend on testing your app?
I could provide you with information that would assist your testing effort if you could provide additional information. Please consider calling me. Failing that, attempt to rephrase the question, including some non-revealing details.