I'm not sure that per-request would work; because the resources that a request takes can vary wildly in resources used / time taken. PiCloud (somewhat similar idea) used to charge based on processing time essentially (down to the millisecond I believe).
I personally think that is the correct kind of pricing for something like this; but monthly plans including X time/requests would likely be a good idea.
I personally think that is the correct kind of pricing for something like this; but monthly plans including X time/requests would likely be a good idea.