I've got to figure $X/mo is way better than enter your API key.
I thought it was a hoot when I was making a paid API about 10 years ago and there were zillions of API gateway products out there that solved all sorts of problems I didn't care about but none of them solved the one critical problem of an API business which is "take payments for API calls". Quite literally you do not have a business if you don't have that, everything else like documentation generators is optional.
as a dev, I don't want to deal with tax reporting, sales tax calculation. I am concerned that an unexpected bill, caused by my users abusing my service, may have a negative impact on my finances.