When I click on "My Account" there are columns stating the number of credits I have and the total number of books I have posted. There is also a column stating "money saved".  I am not sure how this is calculated. It is supposedly based on $4.50 per book.  Could someone explain the formula used??

If you hover over the "money saved" with your mouse - you'll see that the amount is based on the average used price of a book - $4.50.  Not sure where they came up with that number, but basically, every time you receive a book, they add 4.50 to your total.

My guess at the logic is something like this:

New price of a typical trade paperback = $16.   (MMPs are cheaper, but presumably balanced by the more expensive hardcovers)

Typical used bookstore price for a book is 50% of cover, so typical used bookstore cost = $8

Cost of a PBS credit: $3.49.    $8 - $3.49 = $4.51