I want to sell my app as a one time purchase (not SaaS) but I also need my app to access backend services which end up costing me based on usage. How do I initially price my app without knowing average user usage so that my costs are less than my revenue?
If the regular use of the app costs you money then you may need to think of a different pricing strategy. Might be hard to sell the app once and it to continue to cost you money regularly.
If you do this then the pricing could be tricky. You would have to do some estimations on usage and cost over time. If it costs you $1 per user a month on average that would be $12/yr... expected usage 1-2 years? You just need to build that into the price.
One thing that might help is to release feature editions every 6 - 12 months and charge for the upgrades.
Or instead of charging one time feed charge a reasonable yearly subscription free is ideal.
If development and support for the app are the major expenses, and the backend expenses are relatively minor, you could see if advertising (which probably would scale with backend resource usage nicely) is a possible fit.