I think you could do both and incentivize group call buys, setting X sets per call and then lets say 10 000 calls you reduce the per API cost, so you don't push away anyone but incentives are there for bigger projects to scale

Obviously, the service provider would need to work out the cost based on their servers and stuff.

Like for me i use Supermetrics a lot for my clients and the pricing structure is so rigid it makes reporting so expensive, and I think costs can really be driven down with Bitcoin