> What I'm proposing is adding term metadata that allow the information about pricing to travel with the picture like any other piece of metadata.

This is a fundamentally naive view of the problem: the rates will vary over time and based on the usage and photographers will charge more for the front-page of a major magazine than their kid's PTA newsletter.

The solution is, again, a URL which allows all of those details be conveyed without trying to cram a soon-to-be-stale copy into the photo metadata.