Is the revenue plan based on a monthly subscription model? If not, then I don't know why you'd want to think in terms of "X USD per month"[1].
That being said, kudos, the app looks really fun. It seems to me that you should charge the one-time d/l fee ($0.99 USD)
1. "After 10 months of work, we have 100 days to get our unreleased app to 15K USD per month." [http://launchdiary.postagon.com/]