I can see some problems with your "per-exception" pricing model.
If your product works and adds value, it will result in the reduction of exceptions. A reduction in the number of exceptions will result in web sites requiring a smaller plan. Smaller plans will result in lower revenue. The more useful your app is, the less money you will make (a bit of a generalization but you get the point).
Could you not migrate to page loads? Fire off an event on every page, regardless of whether or not there was an exception?
If your product works and adds value, it will result in the reduction of exceptions. A reduction in the number of exceptions will result in web sites requiring a smaller plan. Smaller plans will result in lower revenue. The more useful your app is, the less money you will make (a bit of a generalization but you get the point).
Could you not migrate to page loads? Fire off an event on every page, regardless of whether or not there was an exception?