I can ask users for a one-time charge to sign up on the app. Also, maybe I can add some "pro" features which I am unsure of, at the moment. Not really sure to be honest, let's see.
My suggestion would be to put this in a FAQ somewhere. I'd imagine to the great majority of people this wouldn't be a big deal, but it would help explain the motivation and, for some people, evaluate trustworthiness.