I really dislike pricing per API call, because I'm vulnerable to bugs or attacks (on my or your side) that can cause the pricing to blow up.

Also, I have the impression of getting screwed because there is no relationship between my usage and your cost (unless your API call is a call to a really complex task, 1000 calls will cost you mere pennies in equivalent CPU time, so the more I use your service the bigger your relative margin!).

The only place I am happy to pay per use is for cloud services, where I rent CPU and I get billed per time used.