You should implement something like this:
- User signs up - gets 5 - 10 FREE Screens.
- Once they've used up their FREE screens take them to a page to purchase more screens.
- When they have 1-2 FREE Screens remaining you should have a popup which says they've nearly used their free screens up and to buy more (with one option offering to go to payment page & the other leting the user continue what they were doing).
- On the page where the users can purchase more screens, I wouldn't bother with a monthly subscription plan because that's not what your target audience is interested in.
I'd offer the following options:
100 Screens - $39 ($0.39/Screen)
250 Screens - $95 ($0.38/Screen)
500 Screens - $180 ($0.36/Screen)