After a book is purchased, cataloged and shelved, there are still ongoing costs to maintain our collections. One research article "On the Cost of Keeping a Book" by Paul N. Courant and Matthew Nielsen (2009), suggests it costs $4.26 per book, per year: http://www.clir.org/pubs/reports/pub147/pub147.pdf (hint, skip to page 91 to get to the easy-to-read storage costs estimates chart)
The cost of storage is all about real estate. If you think about how we all wish for more smart classrooms and collaborative spaces for student research, and then think about the number of books held by the College Libraries that don't circulate and aren't useful to student research, then it becomes clearer why thoughtful collection building is so important.
Beyond physical storage, here is a list of other ongoing activities and costs or maintaining a book on the shelf, including those that get zero or low use:
These are just a few of the activities that contribute to the cost of maintaining our collections. Reduce the size of the collections, and staff will have to spend less time shifting, relabeling, repairing and doing database maintenance.