I just read an interesting case study for metered access to online content in Serials: The Journal for the Serials Community
Pay-per-view article access: a viable replacement for subscriptions?
Apparently Trinity saved more money than they planned to save. Some people who I respect have for a number of years now held the belief that a shift to use based pricing for online journals. But the community of libraries and publishers know that the short term outcome of such a shift would need to be relatively revenue neutral. (As the shift from print to online was. And some publishers created insurance of this during the transition by basing online pricing on past print spend at least for the transition.)
The Trinity case shows an example where the total spend with Elsevier declined by even more than was intended. They attribute this in part to who at Trinity was authorized (students has to ask a librarian). This was not a revenue neutral transition. And it was also not access neutral. The students effectively lost access.
There really is a fixed pool of money available for serials. This can shift from publisher to publisher or bypass the library by going into open access fees but the academic market is not apt to grow much (in North America or Europe) and may well collapse (I may post about his in the future. ) So for the short term any transition to use based pricing needs to be modeled in an environment that has a relatively stable equilibrium. I do not believe metered billing can be introduced on a large scale in a way that will provide the kinds of guarantees that are needed to change the business model generally.
I have in the past been involved with efforts to implement metered billing and believe that as a business model it is an effective way for publishers to compete for a libraries last dollar but not its first.