Here's an example for a brief statement to put in your course syllabus if you're using OER or other free materials, from Rajiv Jhangiani of Brock University.
Course Readings:
Here’s the thing: I don’t want you to spend $200 on a textbook if I can avoid it. And I can avoid it, because we will use a free and open textbook. It is available here in a variety of digital formats (entirely free for you to use or print as needed), but if you prefer you can also order a print copy. It is very important to me that higher education be affordable to any student who wishes to access it.