I have done pricing for multiple AWS services and while many people may be against the per-call pricing model of AWS, I would recommend you do a detailed costing of your service first.

Start with initial assumptions, like amount of customer data eat and then base your calculations on how much data is being requested. You will also get a deep understanding of how your system scales and where the cost really lies. For example, even for a simple crud app running in AWS, you can pretty detailed on your costing. Like how much IO you will consume from RDS per million queries etc.

That will tell you what your margins can be.