As far as I can see there is no clear pricing once you go beyond the free tier - it's contact sales.

This is actually a classic enterprise software approach - as there are no fixed per unit production costs to cover, the pricing is very elastic.

ie each license is probably a negotiation - if you see your app for a one off fee then they want a one-off cut - if you sell a subscription they may want a cut of that instead.

Obviously the risk is if you bake it in and your app becomes successful they may want a larger cut than you can afford, but if they don't negotiate in good faith they won't last very long.