Price per request is the most obvious one.

If you had a widget solver, you could charge per widget solved. This way a one widget solve request or a csv upload of a million widgets are priced accordingly.

You can also charge per mb, if your service is some sort of data piping. Or if it's a ML translation service, you charge per word.

That kinda thing.

Sorry, just seeing this now. In my scenario, it's number of vesseils available for charters. So you pay so much for first vessels and some more for additional vessels. Companies with more vessels have more revenue so they can pay more.