Ask HN: A payment model similar to Google's App Engine?

I'm building an application that will be able to register events from companies/bars/theaters etc. that are interested in exposing their events to a larger audience.

If a user finds an event interesting, he/she can favourite it or save it in their calendar. Once the user does this, it's a +1 count on the number of people/potential visitors for that event.

On the business side of things, I'm wondering if something like the GAE payment method would be a good idea.

That is: allowing the company that posts the event 20 'potential visitors' for free (a free quota), and for every other visitor above these 20, they would have to pay something like 5 cents/visitor. That virtual money would be coming from a balance in which they deposit money via paypal or something. When that deposit runs out, the event goes offline (is not shown to the users anymore).

Has anyone (well...except Google) tried this with any success in their business? Is it a good/bad idea? Any shortcomings? Do you have any suggestions or recommendations with regards to this plan of payment?

The reason I want to offer this quota is for small, independent events to be noticed for free...at least for the first 20 people.

Thanks!