See the page on Google Analytics also.
Estimating cost savings is a difficult task with open textbooks and other publications. Below is a snapshot of the algorithm we use to predict cost savings.
Cost per page (default = $0.10) * Words in the chapter / Words per page (default = 500)
Chapter cost * (Predicted read count + (Untracked views * Reading likelihood) + PDF downloads)
(Book cost * PDF downloads) + Σ(Chapter savings)