We've been tracking that in the context of the bookstore as well. In their analysis, they think that about $500,000 has become open access, and therefore it's not revenue for the bookstore anymore. The university tracks that on a site called Open UBC. They estimate that UBC students saved $4.7 million to around $6 million last year in the adoption of open learning materials.
There's a lot more data on that site that might be of interest to you as well.