Ask HN: What kind of payment methods do devs like?

We have a webservice that is currently free but we're introducing paid components, which include additional API calls as well as data export functionality. We're thinking about pricing and I wanted to get some input from devs. What kind of pricing model is most appealing to you?

- Flat monthly fee (all you can eat)

- Fixed fee per action / API call (pay as you go)

- Credit system, like on istockphoto.com (for example, API calls cost 1 credit, data export costs 5 credits, you buy credits in increments of 10)

Any other suggestions (other than free!) are appreciated.